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Flow Chart for collecting/processing GPS and Gravity Data

1. Field Work: Collect relative gravity observations, tying to the base station every three hours. Get at least 10 minutes of carrier phase GPS at each station (4-5 if you are getting the real-time correction). you might want to use Quickplan to make sure PDOP is going to be below the PDOP mask or you may get some unsuccessful points. Try to occupy a benchmark or two (more if available) in your survey; do a couple repeat observations. Your notes have date, time, location, gravimeter reading, and GPS file name. If there is significant terrain nearby (cliff, roadcut, etc.) record it. If you are doing very detailed gravity record the nearby terrain for the first couple of zones in a Hammer chart. Check out our spreadsheet for gravity data collection and the Hammer chart for inner zones. The Science Complex Base Station gravity is: 980,432.210 mgal on the north edge of the concrete pier/pillar in my lab. The national Imagery and Mapping Agency has a database of US gravity base stations (Local Base Stations 5/02).

2. Download your GPS observations from the data recorder and download base station data from the USFS web site or the Missoula County Site. The USFS base station used to give you results that were 1.16 meters lower than the County Station (12/97). Thus you had to add a column to your spreadsheet that added 1.16 meters to your elevations. Keep this in mind when you select a base station and collect a few points at benchmarks to check it. Record all the steps you take.

3. Differentially correct your GPS readings with Pathfinder Office whether you use the PRO-XRS systems or the Geoexplorers. Set your export parameters so that you get State Plane coordinates (NAD83) with height above ellipsoid and height above mean sea level (MSL in export parameters) in meters using GEOID96 as the datum. Remember to correct for the difference in elevation between the GPS antennae and the ground.

(If the previous steps and export went OK, you can skip this. However, if you ever need to convert among datums, convert a single point from/to lat/long-UTM, etc. then CORPSCON is the way to go (it also comes in a DOS version). You can acquire CORPSCON from the army corp of engineers. If you only exported longitude and latitude, or UTM, you can use CORPSCON to project your latitudes and longitudes into state-plane coordinates. Be careful to get X, Y, latitude and longitude for input/output in the correct columns. You can use GEOID96.EXE to get the geoid corrections if you want to map them in a region. If you are doing your own geoid corrections to HAE data, subtract the geoid correction (which is a negative number) from the GPS' HAE value to get orthometric height)

4. Calculate your drift curve and make standard gravity corrections (theoretical, free air, Bouguer slab) to calculate the simple Bouguer anomaly. Look at, and think carefully about, the results. Cut and paste these formulae into your spreadsheet:

1. For your theoretical gravity correction use: gth = 9.7803267714*((1 + 0.00193185138639*SIN²(Lat))/(sqrt(1- 0.00669437999013*SIN²(Lat))))* (m/s²)
2. For free air corrections use 0.3086 mgals/m (0.09406 mgals/foot).
3. For Bouguer slab corrections (@2670 kg/m ³) use 0.11195 mgals/m (0.03412 mgals/ft).
*Blakely, 1995, Potential Theory in Gravity and Magnetic Applications, Cambridge Univ. Press, 441 p. This is for the recent, 1984, Geodetic Reference System adopted by the International Association of 5. Geodicists and corresponds to the WGS84 datum.

The theoretical, free air, and Bouguer slab corrections should use NAD83 locations (long/lat) with elevations from the GEOID96 datum (height above mean sea level using GEOID96) as described above.

5. Transform your vertical coordinates to NGVD29. This is a CORPSCON operation and should yield a new column in your spreadsheet; we'll go over it and terrain corrections some rainy day. The height above mean sea level elevations using GEOID96 comes out of Pathfinder Office in NAVD88 for the vertical datum (even though Trimble doesn't tell you that). You need to convert your mean sea level elevations from NAVD88 (aka GEOID96 MSL) to NGVD29. You have to do this because we will be using state digital elevation models (DEMs) and those are in NGVD29 coordinates as are 7.5 minute topographic maps. Thus you need NGVD29 elevations for terrain corrections but your final maps should be in the NAD83 horizontal coordinates with vertical coordinates in NAVD88 to match Evans (1998). For reference, WGS84 is essentially the same as GRS80 and NAD83 for horizontal and HAE coordinates.

6. Start your terrain corrections by getting or making an ASCII (x, y, z) file for the terrain. The state library has 30-meter DEMs for ½ degree areas. I accumulated, collated, and filtered some local data for terrain corrections close to Missoula. If you make your own elevation files, you should usually use 30-meter samples within a couple kilometers, then 100 meter, then 250 meter samples as you get out to 20 kilometers or more. If you need precise terrain corrections you should always survey or estimate the nearby terrain for the inner zones of the Hammer terrain chart.

7. Use HAMXYZ2.EXE to make terrain corrections for your data. Add the terrain corrections to your simple Bouguer anomalies to get complete Bouguer anomalies. After terrain corrections, go back to NAVD88 for vertical coordinates.

8. Create your final spreadsheet, which we might want to merge with the others in class, with these columns, in this order:

Yr _Name

Montana State Plane

NVGD29

Correct USFS error

Elevation corrections (except TC) are done in NAVD88 to match Evans.

Long

Lat

Easting

Northing

HAE

HAG

HAG+1.16m

gobs

gth

FAC

FAA

CBA

Longitude and Latitude are in decimal degrees, Easting, Northing, Geoid correction and elevations are in meters, gravity values are in milligals.

9. Use SURFER to make a contour map of your gravity data and/or use ExCel or Grapher to plot profiles from your spreadsheet. Do all your analysis in WGS84/NAD83 for horizontal coordinates and NAVD88 for vertical.

10. Use GravCadW to do 2D interpretations of your profiles and GI3.EXE for simple 3D interpretation.

11. Write a Report.

12. Check for internal consistency:

Does a map of the terrain from your spreadsheet look like the terrain where you worked?
Was your gridding in rectilinear state-plane or UTM coordinates (not latitude/longitude)?
Compare multiple GPS/gravity observations from the same place: What is the error in your GPS and gravity determinations?
Take two well-separated points in your survey and graph the difference in terrain correction versus increasing radius for the Hammer correction; start distant and successively remove the outermost ring from Hammer.con for several trials. Thus you'll know how far out you need to do terrain corrections for a particular survey.