Where is the Geography Building?

A Study in the Problems of Precise Positioning

Last Seen on 24th Street before Its Disappearance

These materials may be used for study, research, and education, but please credit the author and source: Kenneth E. Foote and Peter H. Dana, The Geographer's Craft Project, Department of Geography, University of Texas at Austin. All commercial rights reserved. Copyright 1996 Kenneth E. Foote and Peter H. Dana. Send comments and suggestions to k.foote@mail.utexas.edu.

1. The Issue

How are precise geographic positions obtained and to what extent is this precision related to source materials and method of measurement?

To answer this question you will map the outline of the Geography Building not once, but many times. Each map will be drafted from a different source including: a 1:25,000 USGS topographic sheet; both paper and digital 1:1,200 University of Texas Physical Plant Engineering Plans; a paper 1:1,200 City of Austin Engineering Map; an air photograph; uncorrected code-phase GPS; differential code-phase GPS; and points surveyed with a total station.

Each of these maps will be drafted into a different layer of a Microstation design file, as noted below. Each new layer will drafted with the others turned off so that you cannot see the differences until later. Your final map will be a composite showing the many different outlines of the Geography Building and its immediate area. The map show be designed to highlight the differences of locational precision resulting from the varying sources employed and will note the range of imprecision involved.

Here we go...

2. Acquire the Digital Campus Plan

A digital copy of the campus plan is available from our anonymous ftp server austin.grg.utexas.edu, and go to the CAMPUS directory. Alternatively, the files can be obtained from the download directory by clicking on each file while holding down the shift key.

This file, utcampus.dgn, contains the entire campus. Level 1 includes all roads and all building, except the outline of the Geography Building. The outline of the Geography Building is in level 2.

The coordinates of this file are SPC 1927, Texas Central Zone. We will use these for all subsequent coverages, converting other coordinates as necessary.

3. Add the Outline from the 1:24,000 USGS Austin East Topographic Sheet to level 3

Make sure levels 1-2 are off.

Register the topo sheet to your digital file using the SPC coordinates listed on the map.

Set the symbology for this tracing--color, line weight, and line type.

Trace the outline of the Geography Building.

4. Add the Outline from the 1:1,200 City of Austin Engineering Map to level 4

Make sure levels 1-3 are off.

Register the engineering map to your digital file using the SPC coordinates found on the map.

Set the symbology for this tracing.

Trace the outline of the Geography Building.

5. Add the Outline from the 1:1,200 University of Texas Engineering Plan of Campus to level 5

Make sure levels 1-4 are off.

Register the engineering map to your digital file using the SPC coordinates found on the map.

Set the symbology for this tracing.

Trace the outline of the Geography Building.

6. Add the Outline from a Rectified Air Photograph to Level 6

Make sure levels 2-5 are off. Level 1 should be on for this part of the exercise.

Register the air photo to your digital file by matching features of the photograph to features on the campus digital file (level 1).

Set the symbology for this tracing.

Trace the outline of the Geography Building.

7. Add the Outline Using Uncorrected Code-Phase GPS to level 7

Instructions for using the department's GPS receivers will be provided in class and in lab. Before making your GPS measurements be sure to read the Notes on the GPS Positioning Exercise for the Geographer's Craft Work in small groups to make the readings after checking out the equipment from Don Davis. The measurements for the uncorrected and differential GPS positions can be made in the same session.

Once you have made the measurements and converted them to NAD 27 SPC:

Turn off levels 1-6.

Set the symbology for this tracing.

To add the outline of the Geography Building to your file, "key-in" the coordinates in the Microstation command window or use the "Precision Input" menu.

8. Add the Outline Using Differential Code-Phase GPS to level 8

Instructions for calculating differential GPS positions will be provided in class and in lab. For differential corrections, we will acquire date from a GPS basestation maintained by Miller Blueprint.

Once you have made the measurements and converted them to NAD 27 SPC:

Turn off levels 1-7.

Set the symbology for this tracing.

To add the outline of the Geography Building to your file, "key-in" the coordinates in the Microstation command window or use the "Precision Input" menu.

9. Add the Outline Using Measurements from a Total Station to level 9

Instructions for using the total station will be provided in class and in lab. You will work in small groups to perform the measurements from a survey point behind the Geography Building. You will "shoot" three of the four corner points of the building.

Once you have made the measurements and converted them to NAD 27 SPC:

Turn off levels 1-8.

Set the symbology for this tracing.

To add the outline of the Geography Building to your file, "key-in" the coordinates in the Microstation command window or use the "Precision Input" menu.

10. Calculate the Range of Differences in Position

Now, look at levels 1-9 together. For each level, use Microstation's measurement tools to find the difference between the tracing in that level and the coordinates found in level 2. Average these differences for each level to find the range of imprecision in the composite map. How is accuracy related to the source map and method of measurement?

11. Composing Your Final Map

Develop a thematic map that highlights the range of differences in your tracings of the Geography Building. You may wish to clip out a small section of Level 1 (the entire campus map) directly around the building and place this in the visual background. Highlight your tracings with color, line weight, and line type. Annotate the tracings to indicate the sources and differences among the levels.

12. What to Hand In

Please hand in a single paper map that displays the different tracings of the Geography Building and highlights the range of discrepancies between the source measurements.

Last revised 26 January 2000. LNC.