MODULE location_mod (threed_sphere)

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The DART framework needs to be able to compute distances between locations, to pass location information to and from the model interface code (model_mod.f90), and to be able to read and write location information to files. DART isolates all this location information into separate modules so that the main algorithms can operate with the same code independent of whether the model uses latitude/longitude/height, 1D unit sphere coordinates, cylindrical coordinates, etc. DART provides about half a dozen possible coordinate systems, and others can be added. The most common one for geophysical models is this one: threed_sphere.

This location module provides a representation of a physical location on a 3-D spherical shell, using latitude and longitude plus a vertical component with choices of vertical coordinate type such as pressure or height in meters. A type that abstracts the location is provided along with operators to set, get, read, write, and compute great circle distances between locations. This is a member of a class of similar location modules that provide the same abstraction for different represenations of physical space.



The location routines are general purpose code that can be used for a variety of utilities. The following discussion is specifically restricted to how the location namelist settings affect the execution of the filter assimilation program.

Issues related to changing the results of an assimilation based on the location module settings:

Issues related to changing the results of an assimilation based on code in the model-specific model_mod.f90 module:

Issues related to the speed/efficiency of an assimilation based on the location module settings:

Vertical Issues

The localization distance during an assimilation -- the maximum separation between an observation and a state vector item potentially affected by the assimilation -- is set in the &assim_tools_nml namelist (the cutoff item).

Setting horiz_dist_only = .TRUE. in the namelist means the great circle distances will be computed using only the latitude and longitudes of the two locations, ignoring the vertical components of the locations. The cutoff is specified in radians to be independent of the radius of the sphere. For the Earth the radius is nominally 6,371 Km. To compute the horizontal only localization distance, multiply 6,371 Km by the cutoff to get the distance in Km. The cutoff is by definition 1/2 the distance to where the increments go to 0, so multiply that result by 2 to get the maximum distance at which an observation can alter the state.

Setting horiz_dist_only = .FALSE. in the namelist means the code will compute a 3D distance, including the vertical separation. In this case, the vert_normalization_xxx namelist values will be used to convert from pressure, height, model level, or scale heights into radians so the distances are computed in a consistent unit system. In practice, multiply the cutoff by the normalization factor (and then again by 2) to get the maximum vertical separation in each of the given units.

When including vertical separation the potential area of influence of an assimilated observation is an ellipsoid with the observation at the center. The horizontal radius is defined by the cutoff and the vertical radius is defined by the normalization factors.

Different vertical factors per observation type

Generally a single cutoff value and a single set of normalization factors are sufficient for most assimilations. The localization distances define the maximum range of impact of an observation, but there still must be a positive or negative correlation between the state ensemble and the forward operator/expected obs ensemble for the values to change.

However, the &assim_tools_nml namelist includes the option to set a different cutoff on a per-observation-type basis. There are corresponding items in the location module namelist to similiarly control the vertical distance contribution on a per-observation, per-vertical-type basis.

Model-dependent Vertical Conversion Issues

If the model supports either a different vertical coordinate than the vertical coordinate of the observations, or if the user wants to localize in a different vertical coordinate than the observations or state vector items, the model-specific model_mod.f90 code will have to provide a conversion between different vertical coordinates. This cannot be done by the location module since most vertical conversions require additional model-specific information such as temperature, moisture, depth, surface elevation, model levels, etc.

Once the locations have the same vertical units the location module code can compute the distances between them. It is an error to ask for the distance between two locations with different vertical coordinates unless you have set the namelist to horizontal distances only.

There is a vertical type of VERTISUNDEF (Vertical is Undefined). This is used to describe observations where there is no specific single vertical location, for example the position of a hurricane or a column integrated quantity. In this case the location code computes only horizontal distances between any pair of observations in which one or both have an undefined vertical location.

Model-dependent Distance Adjustments

The calls to routines that collect the distances between locations for the assimilation code pass through the model-specific model_mod.f90 code. This allows the code to alter the actual distances to either increase or decrease the effect of an observation on the state or on other observations.

For example, if the top of a model is externally constrained then modifications by the assimilation code may lead to bad results. The model-specific code can compute the actual distances between two locations and then increase it artificially as you reach the top of the model, so observations have smaller and smaller impacts. For ocean models, the distances to points on land can be set to a very large value and those points will be unaffected by the assimilation.

Approximate distances

For regional models this should usually be .FALSE. in the namelist.

For global models this is usually set to .TRUE. which allows the code to run slightly faster by precomputing tables of sines, cosines, and arc cosines used in the distance computations. Values are linearly interpolated between entries in the table which leads to minor roundoff errors. These are negligible in a global model but may be significant in models that over a small region of the globe.

Internal bin counts

The default settings for nlon and nlat are usually sufficient. However if this is a high resolution model with a large state vector the assimilation may run faster by doubling these values or multiplying them by 4. (The nlon item must be odd; compute the value and subtract 1.) These values set the number of internal bins used inside the code to pre-sort locations and make it faster to retrieve all locations close to another location. A larger bin count uses more memory but shortens the linear part of the location search.



This namelist is read from the file input.nml. Namelists start with an ampersand '&' and terminate with a slash '/'. Character strings that contain a '/' must be enclosed in quotes to prevent them from prematurely terminating the namelist.

    horiz_dist_only                          = .true.
    vert_normalization_pressure              = 100000.0
    vert_normalization_height                = 10000.0
    vert_normalization_level                 = 20.0
    vert_normalization_scale_height          = 5.0
    approximate_distance                     = .false.
    nlon                                     = 71
    nlat                                     = 36
    output_box_info                          = .false.
    print_box_level                          = 0
    special_vert_normalization_obs_types     = 'null'
    special_vert_normalization_pressures     = -888888.0
    special_vert_normalization_heights       = -888888.0
    special_vert_normalization_levels        = -888888.0
    special_vert_normalization_scale_heights = -888888.0

Items in this namelist either control the way in which distances are computed and/or influence the code performance.

Item Type Description
horiz_dist_only logical If .TRUE. compute great-circle distance using the horizontal distance component only. If .FALSE. compute distances by including the vertical and horizontal separation. All distances are computed in radians; the corresponding vertical normalization factors are used to compute the vertical distance.
The vertical coordinate system must be the same for both locations in order to compute a distance. However, if either location is VERTISUNDEF, or both are VERTISSURFACE, only a horizontal distance is computed. For any other combination of vertical coordinate systems this routine will fail because it cannot convert between vertical coordinate systems without model-specific information. The model_mod interface code may supply a get_close_obs() routine to intercept and convert the vertical coordinates before calling this get_close_obs() routine.
vert_normalization_pressure real(r8) The number of pascals equivalent to a horizontal distance of one radian.
vert_normalization_height real(r8) The number of meters equivalent to a horizontal distance of one radian.
vert_normalization_level real(r8) The number of model levels equivalent to a horizontal distance of one radian.
vert_normalization_scale_height real(r8) The number of scale heights equivalent to a horizontal distance of one radian.
approximate_distance logical If true, uses a table lookup for fast approximate computation of distances on sphere. Distance computation can be a first order cost for some spherical problems so this can increase speed significantly at a loss of some precision. WARNING: This should be set to .FALSE. if you need to compute small distances accurately or you have a regional model.
nlon integer Used internally by the search code to speed the search for nearby locations. Number of boxes (bins) created in the longitude direction. Must be an odd number. (See discussion above for more information about this item.)
nlat integer Used internally by the search code to speed the search for nearby locations. Number of boxes (bins) created in the latitude direction. (See discussion above for more information about this item.)
output_box_info logical If true, print details about the distribution of locations across the array of boxes. print_box_level controls how much detail is printed.
print_box_level integer If output_box_info = .true., print_box_level controls how much detail is printed. 0 = no detail. 1,2,3 are progressively more and more detail.
special_vert_normalization_obs_types character(len=32), dimension(500) If specified, must be a string array of observation specific types (e.g. RADIOSONDE_TEMPERATURE, AIRCRAFT_TEMPERATURE, etc). For each type listed here a vertical normalization value must be given which overrides the default vertical normalization values. Even if only one is going to be used, all 4 normalization values must be specified for each special type.
special_vert_normalization_pressure real(r8), dimension(500) The number of pascals equivalent to a horizontal distance of one radian, one value for each special observation type listed in the 'special_vert_normalization_obs_types' list.
special_vert_normalization_height real(r8), dimension(500) The number of geopotential meters equivalent to a horizontal distance of one radian, one value for each special observation type listed in the 'special_vert_normalization_obs_types' list.
special_vert_normalization_scale_height real(r8), dimension(500) The number of scale heights equivalent to a horizontal distance of one radian, one value for each special observation type listed in the 'special_vert_normalization_obs_types' list.
special_vert_normalization_level real(r8), dimension(500) The number of model levels equivalent to a horizontal distance of one radian, one value for each special observation type listed in the 'special_vert_normalization_obs_types' list.



Location-independent code

All types of location modules define the same module name location_mod. Therefore, the DART framework and any user code should include a Fortran 90 use statement of location_mod. The selection of which location module will be compiled into the program is controlled by which source file name is specified in the path_names_xxx file, which is used by the mkmf_xxx scripts.

All types of location modules define the same Fortran 90 derived type location_type. Programs that need to pass location information to subroutines but do not need to interpret the contents can declare, receive, and pass this derived type around in their code independent of which location module is specified at compile time. Model and location-independent utilities should be written in this way. However, as soon as the contents of the location type needs to be accessed by user code then it becomes dependent on the exact type of location module that it is compiled with.

Usage of distance routines

Regardless of the fact that the distance subroutine names include the string 'obs', there is nothing specific to observations in these routines. They work to compute distances between any set of locations. The most frequent use of these routines in the filter code is to compute the distance between a single observation and items in the state vector, and also between a single observation and other nearby observations. However, any source for locations is supported.

In simpler location modules (like the oned version) there is no need for anything other than a brute force search between the base location and all available state vector locations. However in the case of large geophysical models which typically use the threed_sphere locations code, the brute-force search time is prohibitive. The location code pre-processes all locations into a set of bins and then only needs to search the lists of locations in nearby bins when looking for locations that are within a specified distance.

The expected calling sequence of the get_close routines is as follows:

call get_close_maxdist_init()  ! is called before get_close_obs_init()
call get_close_obs_init()
call get_close_obs()           ! called many, many times
call get_close_obs_destroy()

In the threed_sphere implementation the first routine initializes some data structures, the second one bins up the list of locations, and then the third one is called multiple times to find all locations within a given radius of some reference location, and to optionally compute the exact separation distance from the reference location. The last routine deallocates the space. See the documentation below for the specific details for each routine.

All 4 of these routines must be present in every location module but in most other versions all but get_close_obs() are stubs. In this threed_sphere version of the locations module all are fully implemented.

Interaction with model_mod.f90 code

The filter and other DART programs could call the get_close routines directly, but typically do not. They declare them (in a use statement) to be in the model_mod module, and all model interface modules are required to supply them. However in many cases the model_mod only needs to contain another use statement declaring them to come from the location_mod module. Thus they 'pass through' the model_mod but the user does not need to provide a subroutine or any code for them.

However, if the model interface code wants to intercept and alter the default behavior of the get_close routines, it is able to. Typically the model_mod still calls the location_mod routines and then adjusts the results before passing them back to the calling code. To do that, the model_mod must be able to call the routines in the location_mod which have the same names as the subroutines it is providing. To allow the compiler to distinguish which routine is to be called where, we use the Fortran 90 feature which allows a module routine to be renamed in the use statement. For example, a common case is for the model_mod to want to supply additions to the get_close_obs() routine only. At the top of the model_mod code it would declare:

use location_mod, only :: location_get_close_obs => get_close_obs,    &
                          get_close_maxdist_init, get_close_obs_init, &

That makes calls to the maxdist_init, init, and destroy routines simply pass through to the code in the location_mod, but the model_mod must supply a get_close_obs() subroutine. When it wants to call the code in the location_mod it calls location_get_close_obs().

One use pattern is for the model_mod to call the location get_close_obs() routine without the dist argument. This returns a list of any potentially close locations without computing the exact distance from the base location. At this point the list of locations is a copy and the model_mod routine is free to alter the list in any way it chooses: it can change the locations to make certain types of locations appear closer or further away from the base location; it can convert the vertical coordinates into a common coordinate type so that calls to the get_dist() routine can do full 3d distance computations and not just 2d (the vertical coordinates must match between the base location and the locations in the list in order to compute a 3d distance). Then typically the model_mod code loops over the list calling the get_dist() routine to get the actual distances to be returned to the calling code. To localize in the vertical in a particular unit type, this is the place where the conversion to that vertical unit should be done.

Horizontal Distance Only

If horiz_distance_only is .true. in the namelist then the vertical coordinate is ignored and only the great-circle distance between the two locations is computed, as if they were both on the surface of the sphere.

If horiz_distance_only is .false. in the namelist then the appropriate normalization constant determines the relative impact of vertical and horizontal separation. Since only a single localization distance is specified, and the vertical scales might have very different distance characteristics, the vert_normalization_xxx values can be used to scale the vertical appropriately to control the desired influence of observations in the vertical.

Precomputation for Run-time Search Efficiency

For search efficiency all locations are pre-binned. The surface of the sphere is divided up into nlon by nlat boxes and the index numbers of all items (both state vector entries and observations) are stored in the appropriate box. To locate all points close to a given location, only the locations listed in the boxes within the search radius must be checked. This speeds up the computations, for example, when localization controls which state vector items are impacted by any given observation. The search radius is the localization distance and only those state vector items in boxes closer than the radius to the observation location are processed.

The default values have given good performance on many of our existing model runs, but for tuning purposes the box counts have been added to the namelist to allow adjustment. By default the code prints some summary information about how full the average box is, how many are empty, and how many items were in the box with the largest count. The namelist value output_box_info can be set to .true. to get even more information about the box statistics. The best performance will be obtained somewhere between two extremes; the worst extreme is all the points are located in just a few boxes. This degenerates into a (slow) linear search through the index list. The other extreme is a large number of empty or sparsely filled boxes. The overhead of creating, managing, and searching a long list of boxes will impact performance. The best performance lies somewhere in the middle, where each box contains a reasonable number of values, more or less evenly distributed across boxes. The absolute numbers for best performance will certainly vary from case to case.

For latitude, the nlat boxes are distributed evenly across the actual extents of the data. (Locations are in radians, so the maximum limits are the poles at -PI/2 and +PI/2). For longitude, the code automatically determines if the data is spread around more than half the sphere, and if so, the boxes are distributed evenly across the entire sphere (longitude range 0 to 2*PI). If the data spans less than half the sphere in longitude, the actual extent of the data is determined (including correctly handling the cyclic boundary at 0) and the boxes are distributed only within the data extent. This simplifies the actual distance calculations since the distance from the minimum longitude box to the maximum latitude box cannot be shorter going the other way around the sphere. In practice, for a global model the boxes are evenly distributed across the entire surface of the sphere. For local or regional models, the boxes are distributed only across the the extent of the local grid.

For efficiency in the case where the boxes span less than half the globe, the 3D location module needs to be able to determine the greatest longitude difference between a base point at latitude φs and all points that are separated from that point by a central angle of θ. We might also want to know the latitude, φf , at which the largest separation occurs. Note also that an intermediate form below allows the computation of the maximum longitude difference at a particular latitude.

The central angle between a point at latitude φs and a second point at latitude φf that are separated in longitude by Δλ is

  θ = cos-1(sinφssinφf + cosφscosφfcosΔλ)

Taking the cos of both sides gives

  cosθ = (sinφssinφf + cosφscosφfcosΔλ)

Solving for cosΔλ gives

  cosΔλ = (a - b sinφf)/(c cosφf) = a/c secφf - b/c tanφf

where a = cosθ , b = sinφs , and c = cosφs . We want to maximize Δλ which implies minimizing cosΔλ subject to constraints. Taking the derivative with respect to φf gives

  (d cosΔλ)/(dφf) = a/c secφf tanφf - b/c sec2φf = 0

Factoring out secφf which can never be 0 and using the definitions of sec and tan gives

  (a sinφf)/(c cosφf) - b/(c cosφf) = 0

Solving in the constrained range from 0 to PI/2 gives

   sinφf = b/a = sinφs/cosθ

So knowing base point (φs, λs), latitude φf, and distance θ we can use the great circle equation to find the longitude difference at the greatest separation point

   Δλ = cos-1((a - (b sinφf)) / (c cosφf))

Note that if the angle between the base point and a pole is less than or equal to the central angle, all longitude differences will occur as the pole is approached.




Public Interfaces

use location_mod, only : location_type

Namelist interface &location_nml must be read from file input.nml.

A note about documentation style. Optional arguments are enclosed in brackets [like this].

type location_type
   real(r8) :: lon, lat, vloc
   integer  :: which_vert
end type location_type

Provides an abstract representation of physical location on a three-d spherical shell.

Component Description
lon longitude in radians
lat latitude in radians
vloc vertical location, units as selected by which_vert
which_vert type of vertical location: -2=no specific vert location; -1=surface; 1=level; 2=pressure; 3=height, 4=scale height
The vertical types have parameters defined for them so they can be referenced by name instead of number.

type get_close_type
   integer  :: num
   real(r8) :: maxdist
   integer, pointer :: lon_offset(:, :)
   integer, pointer :: obs_box(:)
   integer, pointer :: count(:, :)
   integer, pointer :: start(:, :)
end type get_close_type

Provides a structure for doing efficient computation of close locations.

Component Description
num Number of locations in list
maxdist Threshhold distance. Anything closer is close.
lon_offset Dimensioned nlon by nlat. For a given offset in longitude boxes and difference in latitudes, gives max distance from base box to a point in offset box.
obs_box Dimensioned num. Gives index of what box each location is in.
count Dimensioned nlon by nlat. Number of obs in each box.
start Dimensioned nlon by nlat. Index in straight storage list where obs in each box start.

var = get_location(loc)
real(r8), dimension(3)          :: get_location
type(location_type), intent(in) :: loc

Extracts the longitude and latitude (converted to degrees) and the vertical location from a location type and returns in a 3 element real array.

get_location The longitude and latitude (in degrees) and vertical location
loc A location type

var = set_location(lon, lat, vert_loc, which_vert)
type(location_type) :: set_location
real(r8), intent(in)    :: lon
real(r8), intent(in)    :: lat
real(r8), intent(in)    :: vert_loc
integer,  intent(in)    :: which_vert

Returns a location type with the input longitude and latitude (input in degrees) and the vertical location of type specified by which_vert.

set_location A location type
lon Longitude in degrees
lat Latitude in degrees
vert_loc Vertical location consistent with which_vert
which_vert The vertical location type

call write_location(locfile, loc [, fform, charstring])
integer,               intent(in)       ::  locfile 
type(location_type),   intent(in)       ::  loc 
character(len=*), optional, intent(in)  ::  fform 
character(len=*), optional, intent(out) ::  charstring 

Given an integer IO channel of an open file and a location, writes the location to this file. The fform argument controls whether write is "FORMATTED" or "UNFORMATTED" with default being formatted. If the final charstring argument is specified, the formatted location information is written to the character string only, and the locfile argument is ignored.

locfile the unit number of an open file.
loc location type to be written.
fform Format specifier ("FORMATTED" or "UNFORMATTED"). Default is "FORMATTED" if not specified.
charstring Character buffer where formatted location string is written if present, and no output is written to the file unit.

var = read_location(locfile [, fform])
type(location_type)                    :: read_location
integer, intent(in)                    :: locfile
character(len=*), optional, intent(in) :: fform

Reads a location_type from a file open on channel locfile using format fform (default is formatted).

read_location Returned location type read from file
locfile Integer channel opened to a file to be read
fform Optional format specifier ("FORMATTED" or "UNFORMATTED"). Default "FORMATTED".

call interactive_location(location [, set_to_default])
type(location_type), intent(out) :: location
logical, optional, intent(in)    :: set_to_default

Use standard input to define a location type. With set_to_default true get one with all elements set to 0.

location Location created from standard input
set_to_default If true, sets all elements of location type to 0

var = query_location(loc [, attr])
real(r8)                               :: query_location
type(location_type), intent(in)        :: loc
character(len=*), optional, intent(in) :: attr

Returns the value of which_vert, latitude, longitude, or vertical location from a location type as selected by the string argument attr. If attr is not present or if it is 'WHICH_VERT', the value of which_vert is converted to real and returned. Otherwise, attr='LON' returns longitude, attr='LAT' returns latitude and attr='VLOC' returns the vertical location.

query_location Returns longitude, latitude, vertical location, or which_vert (converted to real)
loc A location type
attr Selects 'WHICH_VERT', 'LON', 'LAT' or 'VLOC'

var = set_location_missing()
type(location_type) :: set_location_missing

Returns a location with all elements set to missing values defined in types module.

set_location_missing A location with all elements set to missing values

call get_close_maxdist_init(gc,maxdist, [maxdist_list])
type(get_close_type), intent(inout) :: gc
real(r8), intent(in)                :: maxdist
real(r8), intent(in), optional      :: maxdist_list(:)

Sets the threshhold distance. maxdist is in units of radians. Anything closer than this is deemed to be close. This routine must be called first, before the other get_close routines. It allocates space so it is necessary to call get_close_obs_destroy when completely done with getting distances between locations.

If the last optional argument is not specified, maxdist applies to all locations. If the last argument is specified, it must be a list of exactly the length of the number of specific types in the obs_kind_mod.f90 file. This length can be queried with the get_num_types_of_obs() function to get count of obs types. It allows a different maximum distance to be set per base type when get_close() is called.

gc Data for efficiently finding close locations.
maxdist Anything closer than this number of radians is a close location.
maxdist If specified, must be a list of real values. The length of the list must be exactly the same length as the number of observation types defined in the obs_def_kind.f90 file. (See get_num_types_of_obs() to get count of obs types.) The values in this list are used for the obs types as the close distance instead of the maxdist argument.

call get_close_obs_init(gc, num, obs)
type(get_close_type),             intent(inout) :: gc
integer,                          intent(in)    :: num
type(location_type), dimension(:) intent(in)    :: obs

Initialize storage for efficient identification of locations close to a given location. Allocates storage for keeping track of which 'box' each location in the list is in. Must be called after get_close_maxdist_init, and the list of locations here must be the same as the list of locations passed into get_close_obs(). If the list changes, get_close_obs_destroy() must be called, and both the initialization routines must be called again. It allocates space so it is necessary to call get_close_obs_destroy when completely done with getting distances between locations.

gc Structure that contains data to efficiently find locations close to a given location.
num The number of locations in the list.
obs The locations of each element in the list, not used in 1D implementation.

call get_close_obs(gc, base_obs_loc, base_obs_type, obs, obs_kind, num_close, close_ind, dist)
type(get_close_type),              intent(in)  :: gc
type(location_type),               intent(in)  :: base_obs_loc
integer,                           intent(in)  :: base_obs_type
type(location_type), dimension(:), intent(in)  :: obs
integer,             dimension(:), intent(in)  :: obs_kind
integer,                           intent(out) :: num_close
integer,             dimension(:), intent(out) :: close_ind
real(r8), optional,  dimension(:), intent(out) :: dist

Given a single location and a list of other locations, returns the indices of all the locations close to the single one along with the number of these and the distances for the close ones. The list of locations passed in via the obs argument must be identical to the list of obs passed into the most recent call to get_close_obs_init(). If the list of locations of interest changes get_close_obs_destroy() must be called and then the two initialization routines must be called before using get_close_obs() again.

Note that the base location is passed with the specific type associated with that location. The list of potential close locations is matched with a list of generic kinds. This is because in the current usage in the DART system the base location is always associated with an actual observation, which has both a specific type and generic kind. The list of potentially close locations is used both for other observation locations but also for state variable locations which only have a generic kind.

If called without the optional dist argument, all locations that are potentially close are returned, which is likely a superset of the locations that are within the threshold distance specified in the get_close_maxdist_init() call. This can be useful to collect a list of potential locations, and then to convert all the vertical coordinates into one consistent unit (pressure, height in meters, etc), and then the list can be looped over, calling get_dist() directly to get the exact distance, either including vertical or not depending on the setting of horiz_dist_only.

gc Structure to allow efficient identification of locations close to a given location.
base_obs_loc Single given location.
base_obs_type Specific type of the single location.
obs List of locations from which close ones are to be found.
obs_kind Generic kind associated with locations in obs list.
num_close Number of locations close to the given location.
close_ind Indices of those locations that are close.
dist Distance between given location and the close ones identified in close_ind.

call get_close_obs_destroy(gc)
type(get_close_type), intent(inout) :: gc

Releases memory associated with the gc derived type. Must be called whenever the list of locations changes, and then get_close_maxdist_init and get_close_obs_init must be called again with the new locations list.

gc Data for efficiently finding close locations.

var = get_dist(loc1, loc2, [, type1, kind2, no_vert])
real(r8)                        :: get_dist
type(location_type), intent(in) :: loc1
type(location_type), intent(in) :: loc2
integer, optional,   intent(in) :: type1
integer, optional,   intent(in) :: kind2
logical, optional,   intent(in) :: no_vert 

Returns the distance between two locations in radians. If horiz_dist_only is set to .TRUE. in the locations namelist, it computes great circle distance on sphere. If horiz_dist_only is false, then it computes an ellipsoidal distance with the horizontal component as above and the vertical distance determined by the types of the locations and the normalization constants set by the namelist for the different vertical coordinate types. The vertical normalization gives the vertical distance that is equally weighted as a horizontal distance of 1 radian. If no_vert is present, it overrides the value in the namelist and controls whether vertical distance is included or not.

The type and kind arguments are not used by the default location code, but are available to any user-supplied distance routines which want to do specialized calculations based on the types/kinds associated with each of the two locations.

loc1 First of two locations to compute distance between.
loc2 Second of two locations to compute distance between.
type1 DART specific type associated with location 1.
kind2 DART generic kind associated with location 2.
no_vert If true, no vertical component to distance. If false, vertical component is included.
var distance between loc1 and loc2.

var = vert_is_undef(loc)
logical                         :: vert_is_undef
type(location_type), intent(in) :: loc

Returns true if which_vert is set to undefined, else false. The meaning of 'undefined' is specific; it means there is no particular vertical location associated with this type of measurement; for example a column-integrated value.

vert_is_undef Returns true if vertical coordinate is set to undefined.
loc A location type

var = vert_is_surface(loc)
logical                         :: vert_is_surface
type(location_type), intent(in) :: loc

Returns true if which_vert is for surface, else false.

vert_is_surface Returns true if vertical coordinate type is surface
loc A location type

var = vert_is_pressure(loc)
logical                         :: vert_is_pressure
type(location_type), intent(in) :: loc

Returns true if which_vert is for pressure, else false.

vert_is_pressure Returns true if vertical coordinate type is pressure
loc A location type

var = vert_is_scale_height(loc)
logical                         :: vert_is_scale_height
type(location_type), intent(in) :: loc

Returns true if which_vert is for scale_height, else false.

vert_is_scale_height Returns true if vertical coordinate type is scale_height
loc A location type

var = vert_is_level(loc)
logical                         :: vert_is_level
type(location_type), intent(in) :: loc

Returns true if which_vert is for level, else false.

vert_is_level Returns true if vertical coordinate type is level
loc A location type

var = vert_is_height(loc)
logical                         :: vert_is_height
type(location_type), intent(in) :: loc

Returns true if which_vert is for height, else false.

vert_is_height Returns true if vertical coordinate type is height
loc A location type

var = has_vertical_localization()
logical :: has_vertical_localization

Returns .TRUE. if the namelist variable horiz_dist_only is .FALSE. meaning that vertical separation between locations is going to be computed by get_dist() and by get_close_obs().

This routine should perhaps be renamed to something like 'using_vertical_for_distance' or something similar. The current use for it is in the localization code inside filter, but that doesn't make this a representative function name. And at least in current usage, returning the opposite setting of the namelist item makes the code read more direct (fewer double negatives).

loc1 == loc2
type(location_type), intent(in) :: loc1, loc2

Returns true if the two location types have identical values, else false.

loc1 /= loc2
type(location_type), intent(in) :: loc1, loc2

Returns true if the two location types do NOT have identical values, else false.

integer, parameter :: VERTISUNDEF       = -2
integer, parameter :: VERTISSURFACE     = -1
integer, parameter :: VERTISLEVEL       =  1
integer, parameter :: VERTISPRESSURE    =  2
integer, parameter :: VERTISHEIGHT      =  3
integer, parameter :: VERTISSCALEHEIGHT =  4

Constant parameters used to differentiate vertical types.

integer, parameter :: LocationDims = 3

This is a constant. Contains the number of real values in a location type. Useful for output routines that must deal transparently with many different location modules.

character(len=129), parameter :: LocationName = "loc3Dsphere"

This is a constant. A parameter to identify this location module in output metadata.

character(len=129), parameter :: LocationLName = 
"threed sphere locations: lon, lat, vertical"

This is a constant. A parameter set to "threed sphere locations: lon, lat, vertical" used to identify this location module in output long name metadata.



filename purpose
input.nml to read the location_mod namelist


  1. none

Error Codes and Conditions

initialize_module nlon must be odd Tuning parameter for number of longitude boxes must be odd for algorithm to function.
get_dist Dont know how to compute vertical distance for unlike vertical coordinates Need same which_vert for distances.
set_location longitude (#) is not within range [0,360] Is it really a longitude?
set_location latitude (#) is not within range [-90,90] Is it really a latitude?
set_location which_vert (#) must be one of -2, -1, 1, 2, 3, or 4 Vertical coordinate type restricted to:
-2 = no specific vertical location
-1 = surface value
1 = (model) level
2 = pressure
3 = height
4 = scale height
read_location Expected location header "loc3d" in input file, got ___ Probable mixing of other location modules in observation sequence processing.
nc_write_location Various NetCDF-f90 interface error messages From one of the NetCDF calls in nc_write_location


The Hawaii and Workshop versions of this module had an error in the approximate distance computation. The available values in the lookup table for cosine were insufficient for some cases. This manifested itself as potential errors, most commonly for computing distances near the poles. For relatively small horizontal localizations, this problem only occurred for locations very near the pole.


Future Plans

Need to provide more efficient algorithms for getting close locations and document the nlon and nlat choices and their impact on cost.

The collection of 'val = vert_is_xxx()' routines should probably be replaced by a single call 'val = vert_is(loc, VERTISxxx)'.

See the note in the 'has_vertical_localization()' about a better name for this routine.

The functions of 'get_close_maxdist_init()' and 'get_close_obs_init()' appear to be able to be combined into a single init routine. This impacts all model_mods, however, since they can intercept these routines. Doing this will be a non-backwards compatible change.

The use of 'obs' in all these routine names should probably be changed to 'loc' since there is no particular dependence that they be observations. They may need to have an associated DART kind, but these routines are used for DART state vector entries so it's misleading to always call them 'obs'.


Private Components



Terms of Use

DART software - Copyright UCAR. This open source software is provided by UCAR, "as is", without charge, subject to all terms of use at

Contact: DART core group
Revision: $Revision: 12417 $
Source: $URL: $
Change Date: $Date: 2018-03-01 16:34:13 -0700 (Thu, 01 Mar 2018) $
Change history:  try "svn log" or "svn diff"