*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**

Distance covered

Number of paces

Number of paces

Number of paces

Average

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.

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.

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.