GEOG 2043 ENVIRONMENTAL FIELD TECHNIQUES LAB 7 GLOBAL POSITIONING SYSTEM (GPS) 2: MAPPING TOPOGRAPHY

OBJECTIVES

• Compare the accuracy of the measurements made with different GPS receivers, and under different conditions.
• Transform the points measured with the GPS into a local coordinate system.
• Plot a simple map of these locations.

We obtained 187 positions at various locations on the landslide in North Boulder.  These data were subsequently processed using differential correction techniques.  The results indicate that most points have a horizontal precision of about 0.3 m, whereas the vertical precision is about 0.6 m- this is about what you would expect for 60 s occupation times.  We will provide you with a copy of the corrected data in spreadsheet format.

TASKS

1. Make two scatter plots, the first showing how horizontal precision varies with PDOP, the second showing how vertical precision varies with PDOP. Be sure to label the axes accordingly.

2. The columns on the far right of the spreadsheet list the longitude and latitude of each position.  These values are listed in degrees and fractional equivalents of a degree.  Using these values, compute the x and y coordinates of each point, relative to a local coordinate system.  Several steps are involved in making these calculations.  First, we define the local coordinate system in terms of the first point.  Then, we adjust the remaining points into a local coordinate system. To envision the process consider the figures below. The figure on the left shows the coordinates in latitude and longitude of two points, A and B, measured with the GPS.  The figure on the right shows the same points, located in in x-y coordinates, assuming that point A is the origin.
 
 

To proceed think about the following questions:

• How would you find the latitude and longitude of point B, given the origin at point A (assuming the curvature of the earth is negligible over these distances)? (note: this doesn't involve any trigonometry)

• How would you convert these locations to coordinate points, measured as distances in meters, from the origin?  (this doesn't involve any trigonometry either)

• Recall that Boulder is at 40 degrees latitude.
• At 40 degrees latitude, the length of one degree of latitude is 111,035 meters and the length of one degree of longitude is 85,394 meters.
• Assuming that the first point is the origin, can you think of a scheme to convert the lat/long of all locations into coordinate points?

4. It turns out that the first point in the data file is the eastern-most point, thus if you've done the computations above correctly, all of the other positions will have negative values in the east-west (X) direction.  This isn't really a problem, but suppose it is necessary to present the data in some other way.  Suppose we want all points to be plotted relative to the western-most point.

• Think of a scheme to transform the locations of all points, such that the western-most point is now the origin.

6. We will hand out copies of maps showing the elevations of points measured in this exercise.  We want you to:

• Hand contour the maps at 10 m intervals

• Hand contour the maps at 5 m intervals

1. Two plots showing trends in horizontal and vertical precision vs. PDOP; one paragraph discussing differences.

2. Spreadsheet calculations showing transformations to local coordinate system.

3. Two plots showing planimetric position of points, one with the eastern-most point as the origin, the other with the western-most point as the origin.

4. Hand-drawn countour maps- ! please take some time with this!  detail and neatness are both important !