SNAP coordinate files can optionally contain information about the gravity field that SNAP can use in an adjustment. This information can include the geoid height (also called the separation or undulation) and the deflection of the vertical.
The geoid information is defined in the station coordinate file. It can be defined explicitly for each station, or implicitly in terms of a vertical datum associated with the coordinate system of the station coordinate file. If the geoid information is defined explicitly for each station then the coordinate system vertical datum is ignored.
The height coordinates of stations in the station coordinate file can be specified as ellipsoidal or orthometric. If this is not specified, then the height type is orthometric if the coordinate system defines an orthometric height system, otherwise they are geoidal. However they may also specified as orthometric heights. The orthometric heights for stations are calculated by subtracting the geoid height from the ellipsoidal height. If there is no geoid information, either explicit or implicit, in the station file then orthometric and ellipsoidal heights are treated as being the same.
snapgeoid is a program to manage the geoid and height information in a station file. It can add or remove explicit geoid information for the stations, or add or remove a vertical datum from the coordinate system. It can also convert the station coordinate height type between ellipsoidal and orthometric heights. Explicit geoid information can be calculated using a vertical datum defined in the coordinate system definition file. It can also be calculated from a binary format gridded geoid model.
Note that when snapgeoid is changing geoid information in a coordinate file it will recalculate the geoid height for each station, which means recalculating the relationship between the orthometric and ellipsoidal heights. Generally when snapgeoid does this it keeps the ellipsoidal height unaltered and updates the orthometric height. The exceptions to this are:
The reason for retaining the ellipsoidal height is that generally converting between orthometric height systems the expectation is that the orthometric height will be changed (for example converting between NZVD2009 and NZVD2016).
The SNAP coordinate system definition file defines the vertical datums that can be used by snapgeoid. Each is reference by a code, for example "NZVD2016". The list of vertical datums can be defined with the -v option.
snapgeoid is a command line utility run from a DOS prompt. The syntax for the command is
snapgeoid -v
snapgeoid [-h vd_code|-g geoid|-c|-z] [options] station_file [new_file]
In this command:
|
station_file |
is the name of the input station coordinate file |
|
new_file |
is the name of the new file that will be created. If this is not specified then it will have the same name as the input file, but with extension ".new". |
The following options can be used with snapgeoid:
|
-v |
If specified then snapgeoid will list the available vertical datum codes and exit |
|
-h vd_code |
Specifies a vertical datum to use to calculate geoid undulations. Use the -v option to output a list of valid codes. |
|
-g geoid |
Specifies a geoid file to use to calculate geoid undulations. |
|
-c |
Instruct snapgeoid to calculate geoid information from the coordinate system defined in the coordinate file. This is only applicable if the coordinate system defines a vertical datum, for example "NZGD2000/NZVD2016". |
|
-z |
Causes snapgeoid to remove (zero) the geoid information from the output coordinate file. If the coordinate system defines a vertical datum this will still apply. |
|
-a vd_code |
Associates a vertical datum with the coordinate system. The height reference surface must be compatible with the coordinate system. If the coordinate file includes explicit geoid information then this surface will not be used by other programs. However it can be used to update the geoid information with the -c option. |
|
-d |
Removes the vertical datum associated with the coordinate system, if any. The vertical datum can be used to update explicit geoid information using the -c option before it is removed. |
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-e |
sets the height coordinate type to ellipsoidal heights. |
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-o |
sets the height type to orthometric heights. |
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-p |
sets the height type to orthometric heights and preserves orthometric heights when the geoid height is changed. Otherwise the ellipsoidal height is preserved, except when the coordinate file does not any geoid information and the height type is orthometric. |
|
-k |
instructs snapgeoid to retain explicit geoid information, overriding the -h, -g, and -c options if the file already had explicit geoid information. |
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-i |
forces ignore errors calculating the geoid height for specific stations. For these stations the geoid information is unaltered. Errors which apply to all stations, such as none existing geoid files or invalid vertical datum code, are still reported. |
|
-q |
reduces the amount of output generated by snapgeoid. |
Note that only one of the -h, -g, -c, and -z options can be used. Similarly onl one of -e, -o, and -p is permitted.