Open/Create Base Station Database

Use the Gravity > Base Stations > Open/Create Base Station Database menu option (GRBASEDB GX) to open or create a base station database that will be used with the current survey project. A base station database contains the absolute gravity and longitude/latitude locations of established gravity base stations that will be used for the current gravity project.

Open/Create a Base Station Database dialog options

Base station database

The name of the base station database. If the database does not exist, it will be created with channels of "Station", "Gravity", "Longitude", "Latitude" and "Elevation". You will also have the option to load existing base station data from another database or a text file.

Application Notes

Gravity surveys are conducted by starting and ending every set of gravity readings at a point with a known absolute gravity value. This point is called a gravity base station. The absolute gravity field at a gravity base station is normally been established by a separate base station survey. Each base station will have the following information:

Station number "Station"	A unique station number that is used to look-up base station parameters during the reduction of survey data.
Absolute gravity "Gravity"	The absolute gravity at the base station in milligals. This value is determined either from some other source (perhaps you are using government established base stations), or by performing a base station survey (See Base Station Surveys below for information on establishing your own base stations), or the gravity is given an arbitrary value.
Longitude, Latitude "Longitude" "Latitude"	The geographic location of the base station. This location is used to establish the earth tide correction. Note that longitudes are negative in the Western hemisphere.
Elevation "Elevation"	This is optional information that is not used in the data reduction. By default, elevation units are assumed to be metres. You can change the elevation units by editing the channel information and setting the unit abbreviation.

If you have established one or more arbitrary gravity base stations, you still need to create a base station database. After you run this GX, enter your base station parameters directly in the base station spreadsheet. The longitude and latitude should be reasonably accurate (within 10 minutes), although the tide correction will be reasonably constant over several degrees. If the precision of the tide correction is a concern, you should test your drift correction for different base station locations to establish an expected error that may result from uncertainties in the base station location.

Base Station Surveys

Gravity survey areas are often located some distance from an existing known gravity base station, or surveys may cover a wide geographic area and you will require more than one gravity base station. If you have access to a known gravity base station, you can establish new base station by conducting a base station survey. If you do not have access to an existing base station, you should create one at a reasonably accessible and stable area and simply specify an arbitrary gravity value for that location. You can then conduct a base station survey to establish other base stations relative to the arbitrarily established base.

A base station survey is conducted just like a normal gravity survey except that all occupied stations are base stations. The new base stations should be identified as type "1" so that they can be processed as a regular survey reading (see the GRDRIFT GX for more information of reading types). The first and last reading in the survey must be on an existing base station (type "0"). The base station survey can be repeated multiple times to improve the accuracy of the absolute gravity by averaging reduced values. All new base stations should be numbered uniquely within your survey project.

After conducting your base station surveys, the data can be drift corrected ("GravRed/Drift correction…", GRDRIFT GX). If more that one base station survey has been conducted, you can merge results from multiple surveys into a single database using the "GravRed/Merge with master database…" (GRAPPEND GX). Once merged, use the "GravRed/Process repeats…" to average repeat values and provide a report on the accuracy of the repeated readings. You should remove all readings from the merged file and leave the averaged readings that contain the absolute gravity values. To remove a reading, mark the line that contains the reading in the left-most column of the spreadsheet and press <Delete>.

Once the "master" base station database is acceptable, it can be merged with the Base Station database using "GravRed/Import/Append base station data…" (GRLDBASE GX).

Script Notes

There is no need to run this GX from a script. The base station name is defined by the GRAVRED.BASE_DATABASE entry in the project. All gravity parameters can be established using the GRPARM GX ("GravRed/Processing parameters…").