Choosing a site:

It is important to know where your stations are, so some care must be taken in locating your sites. If you are using GPS, this is not a big problem, but even so it is wise to have a "backup" means of locating yourself.

If you are doing a regional scale survey (for example, to determine the regional gravity field), pick a site that is easy to locate on the map (like a road intersection). The best of these are are those with elevations shown on the map, the absolute best being bench marks. If no elevation is marked, it is better to stick to a relatively flat area, so that estimates from a map won't be too bad.

If your are doing a small scale survey, locations generally are measured relative to the base station (or some other reference point) and so it is possible to determine these locations using a tape measure, a compass, and a dumpy level.

Taking a gravity reading:

1. Check the temperature of the meter.
   The thermometer at the top of the meter should read about 52 ^oC. If the meter has gone cold, as it will if power has been lost for a few hours, you will need to wait an hour or so after the meter has warmed up to be sure it has stabilized.

2. Set up the base plate.
   If you're on asphalt (or something hard), find a fairly level place to put the base plate. If you're on dirt, push the baseplate in so that it doesn't rock about.

3. Level the meter.
   Usually a good idea is to do a rough level by sliding the meter around in the hyperbolic base plate, as a good course adjustment will save time and frustration. With practice, you can get pretty close to level without adjusting any of the foot screws.

4. Unlock the mass.
   Turn the silver screw near the center of the meter counterclockwise as far as it will go.

5. Turn on the light.
   Flip the toggle switch on the right side of the top face. The telescope (interior) and level lights should go on.

6. Make at least 3 readings.
   When making a reading, move the bar to the right of the scale by turning the nulling dial clockwise, then slowly turn the dial counterclockwise until the left edge of the bar lines up with the registration line (=2.7 for the RPI D Meter). Do this 3 times to help guard agains blunders.

   Alternatively, I've found that the capacitor (galvanometer) readout (the pointer guage on at the upper part of the top face) gives a very accurate reading on the position of the mass. When this is pointing straight up, the mass is nulled.

   Note: Occasionally, the mass will "stick" to one of the stops when it has been locked down, so that it will not move from the side when the nulling dial is turned. If you suspect this is happening, LIGHTLY tap the side of the box to make the mass move.

7. Turn off the light

8. Lock up the mass.
   THIS IS VERY IMPORTANT! Double check to see that the locking screw is turned clockwise as far as it will go before transporting the instrument to the next site.

What to Record in Your Field Notebook:

Start off the notebook with a general description of what you are doing (General location, names of the members in your crew, what the survey is for, the weather, your favorite color). For each station, record the following information:

1. The station name. Usually, names are some prefix followed by a number (for example, STA1, STA2, ..)

2. The time of day. Use 24 hour time (as opposed to AM and PM).

3. The location. For a regional survey, this description should be verbose enough so that anyone can find it on a map. For a local survey, the sites should be measured in.

4. The elevation. For a regional survey, copy the elevation off of the map if it is shown, otherwise provide your best estimage from the contours, Indicate from where the elevation estimate is derived. For a local survey, the sites should be measured in.

5. The three readings, and an average.

6. Local Terrain. If you are not in a flat area, describe the terrain around you. Estimate differences in elevation for features that may not be on evident on a topo map. If you are close to a large change in elevation, measure how far you are from it.

7. Your Comments. Record anything of note about the site. It never hurts to say more, so write as much as you can.

Reducing the Data

Completer gravity reduction, including tidal corrections and terrain corrections, are difficult to do by hand, and probably should wait until you get home and can make use of a program like Xreduce.

Nevertheless, it is useful to keep in mind a few simple approximations to how the gravity field should "normally" vary, in order to see if you are detecting any anomalies and/or if the gravity meter is working properly. The most useful are the changes in gravity with N-S distance and with elevation.

Latitude effect	0.02 mgal/100 feet	Increasing to the North
Elevation effect	0.0341 mgal/ft 0.112 mgal/m	Decreasing with increasing elevation