Latitude Correction

Use the Latitude Correction option (AGLATCOR GX) to calculate latitude corrections.

The option is available with the Gravity and Terrain Correction extension under the following menus:

• Gravity > Gravity Corrections
• Moving Platform Gravity > Corrections by Steps

Calculate Latitude Correction dialog options

Input gravity channel	The input gravity channel containing the gravity readings. Script Parameter AGLATCOR.GRAV
Latitude channel	The input latitude channel. The channel contains the defined latitude ground locations. By default, if there is a channel named "Latitude" in the current database, it will be selected. Script Parameter AGLATCOR.LAT
Latitude correction channel	The latitude correction channel will store Earth's normal gravity as a function of latitude for each station location. The first time through, the default name of this channel is set to "LatCor"; however, you can override the default or select an existing channel (to overwrite) from the list. Script Parameter AGLATCOR.OUTCH
Output gravity channel	The output gravity channel will store the gravity values after removing the latitude correction. The first time through, the default name of this channel is set to "GravCor"; however, you can override the default or select an existing channel (to overwrite) from the list. Script Parameter AGLATCOR.OUTGRAVCOR
Method	Select a gravity latitude correction method. The options are: 1930 - IGF 1967 - IGSN71 1976 - IAG 1980 - GRS80 Moritz 1980 - IAG See the Application Notes below for details on their respective latitude correction formulas and formula customization. Script Parameter AGLATCOR.METHOD

Application Notes

The earth’s gravity field is stronger at the poles than at the equator (by about 5,100 mgals) as a result of several effects. One of these effects is the earth’s ellipsoidal shape. The earth’s surface gets progressively closer to the centre of mass at higher latitudes. As a result, the attraction at the poles is greater than at the equator. The variation in gravity with latitude is defined in terms of gravity on the surface of the spheroid. The latitude correction requires the theoretical gravity at the station location on the earth's spheroid. There are five formulas provided for calculating the latitude correction: 

1. The 1930 International Gravity Formula ^[1] based on the International Ellipsoid used with the Potsdam Ellipsoid:

   

2. The 1967 International Gravity Formula ^[1] based on the Geodetic Reference System used with the IGSN 71 ellipsoid:

   

3. The 1976 Telford formula ^[2]:

   

4. The 1980 (Moritz) formula ^{[1], [4]} (which led to the World Geodetic System 1984):

   

5. The 1980 (Telford) formula ^[3]:

   

Where:

	Theoretical gravity in milligals (latitude correction)
	Latitude of the station

The first time the latitude correction is executed, the five formulas defined using Oasis montaj expressions are saved in the file "Gravity_Latitude.lst" located in the folder %USERPROFILE%\Documents\Geosoft\Desktop Applications \etc.

If you would like to provide alternate equation(s), append them in the "Gravity_Latitude.lst" file.

Each expression should be defined on one line and should consist of a unique name followed by the complete expression using the following syntax:

"Unique_name","@l=$sLatitude/57.29578; LatitudeCorrected = InputGravity - <latitude_correction_equation>"

Where:

• @l is simply a conversion from degrees to radians.
• LatitudeCorrected is the output gravity channel entry.
• InputGravity is the input gravity channel.
• <latitude_correction_equation> is the custom equation and gets saved in the latitude correction channel.

References