Lab exercise: Surveying techniques

Due 3 September, 2002 at the start of class

This exercise should help in appreciating the strengths and weaknesses of some different approaches to locating positions for geophysical field work.

You cannot break the GPS receivers by messing with the buttons. The receivers may be a little intimidating at first and you may be afraid to change settings that you don't understand yet. However, these receivers are built for people who may know very little about the inner workings of GPS. Feel free to play with any of the buttons and functions on the receiver. On the Garmin units, hitting the Page key will usually get you out of almost anything.

1. Learn your pace

Just about the cheapest, fastest way to estimate distances over gentle terrain is by walking. After all, you'll probably have to walk to get to your destination anyway. We usually count paces by counting the steps made with one foot (either right or left, depending on how you start)--this cuts the count down by a factor of two, which can be significant. Lay out a measuring tape. Starting from the end of one tape, walk normally towards the other end, noting the distance of your last stride on the tape. Repeat several times, then sum and divide to get your mean stride. Do NOT try and walk precisely or in some special way--the idea is that your comfortable pace is the one you would use in walking distances appropriate to pacing.

Distance covered

Number of paces

Number of paces

Number of paces

Average

2. Know your compass

You should know how to measure an angular distance with a compass, that is, be able to measure a bearing from north. We usually use a Brunton pocket transit. There are two things to be aware of when getting a bearing with a Brunton:

(1) make sure you have the magnetic declination set correctly. A screw on the side of the instrument adjusts this. In the western U.S., magnetic north is to the east of true north; on a Brunton, this means that the 0 degree mark should be slightly clockwise from the long axis of the compass. In Boulder the declination is just under 11 east of north. (You can check the predicted declination at http://www.hut.fi/%7Ekkorhon1/Java/contourmap.html if you wish; a list of published and other online resources is at http://www.cam.org/~gouletc/decl_faq.html#DETERMINE)

(2) the compass can be affected by local magnetic bodies. Move away from magnetic objects to make measurements. Be aware of magnetic objects you might be carrying (rock hammers, geophysical equipment, cars). If possible, go to a point along your bearing and sight back towards your initial position. Such backsighting will frequently reveal the presence of a local deviation of the magnetic field.

3. Optical measurements.

An important tool for surveying is a total station. It is given this name because it incorporates a distance meter for measuring distances and a theodolite for measuring angles into one instrument. The total station measures by sending a beam of infrared light toward a prism, usually supported either by a tripod or a pole (like I'm holding in the center photo). The light reflects off the prism directly back to the total station.  By measuring the time it takes for the light to return, the total station calculates the distance away that the prism is.  The information that the total station measures (angles and distances)  is recorded in a data collector for later downloading into a computer in the office.  The data collector also doubles as a field computer, enabling us to calculate coordinate geometry in the field.

Surveying Field Assignment

We will be locating various points on the field just south of the Benson Earth Sciences Building to simulate describing a geophysical locality. You will measure distances using  pace and compass route, tape and compass, using a Garmin GPS unit, and using an optical Total station. You should sketch on your field notes the geometry of the field and the approximate position of the points you will be locating. Assign each point some identification (e.g., FF-1) on your sketch. You can do either pace and compass or GPS first.

A. Pace and compass:

For each leg of your journey, note the count of paces (remember the rule of notekeeping: write your raw observations before any calculations), the azimuth (angle of travel relative to north), and the backazimuth (the same as azimuth, except for your return trip from the other end). You will want space for a column for the distance of the traverse. Note at the start the magnetic declination of your compass.

Azimuth Backazimuth Steps Distance
Base to 1
1 to 2
2 to Base

B. GPS:

For each point you will want to note your coordinates (including elevation) and the time (from the GPS unit). Make both a single observation and an observation averaged over a couple of minutes. Again, take care to keep clear notes. Check the datum that your GPS unit is using prior to beginning; write down the datum that you have selected. When you return to your starting point, remeasure and note that location.

Latitude Longitude Elevation
Base
Station 1
Station 2
Base
Distance

Base to 1
1 to 2
2 to Base

C. Tape and compass.

For points identified in the field use the measuring tape and the compass to get the location of these points. You can work with another student on this as wandering with the tape is a little awkward; however, you both should have the measurements in your own notes.

Azimuth Backazimuth Distance
Base to 1
1 to 2
2 to Base

D. Total Station

Use the total station to measure distance and height of the surveying points. Note all measurements and calculations. Use attached sheets for recording total station measurements.

4. Mapping Benson Field: Analysis

From your pace and compass data and your raw GPS measurements (both single measurement and averaged), construct an accurate scale map of the control points around Benson Field, using different colors or symbols for the different techniques. Note elevations for the GPS positions. Details of some of the points might be of use.

When you constructed your pace and compass loop you should ideally have closed the loop (i.e., the vector sum of all your legs should be zero). Calculate the vector sum and discuss its implication for the accuracy of the pace and compass traverse (e.g., what error in distance and azimuth might you expect over 100 feet?).

Similarly, your second GPS location of the starting point should be the same as the first. What difference exists, and what does it tell you of the uncertainty of a GPS location?

Discuss and estimate the accuracy of the different techniques.

Given the difficulty of measurement and the resulting accuracy, suggest a method for locating the two following geophysical surveys, and provide your justification.

(1) a seismic refraction line, consisting of 24 geophones separated by 15 meters apiece and a shotpoint at one end. Locations of geophones to shotpoints need to be known to within about a meter.

(2) a gravity survey of 50 points spread roughly evenly in a 10 km by 20 km rectangle. Gravity stations need to be known to within a meter or so vertically and some 10s of meters north-south to yield a useful measurement.

You will turn in your field notes, including your pace calculation, your annotated scale map, and the discussion of the accuracy of the techniques as outlined above.