CS 128/ES 228 - Introduction to Geographic Information Systems

Lab 13b: GPS - Error Analysis

Goals:

        By the conclusion of this lab period, you will have:

  1. Investigated the effects of differential GPS (DGPS) on positional accuracy.
  2. Produced an analysis of the positional errors in your GPS data set.

Data Analysis

  1. Getting your Garmin data.  Last Monday, the Garmin receiver recorded data in lattitude and longitude.  We used the calculator here to convert these numbers to northings and eastings for zone 17.  The resulting data file is found here.  Of course, there is the chance of a transference error.  Therefore, you should perform a "sanity check" on the data and you should also test that the calculations were done correctly.  Take the sixth reading that your group recorded and place those values into the calculator.  Do they match the values in the spreadsheet?  Were there any other errors?


  2. Getting your Trimble data.  The Trimble receiver's data must be downloaded using the Pathfinder software.  This has also been done for you.  That data is available here.  


  3. Analyzing the data.  We have two fundamental questions:

    1. Ignoring user interface issues, is the more expensive device worth it?
    2. Does the satellite correction improve the accuracy by a significant amount?

    We will answer both of these questions by using a t-test to determine if the errors are significantly different. 
    4. To do so, we must know the precise location of the monuent and the error of the reading. 

    Construct a "composite" spreadsheet as follows.  It should have ten rows of data (plus appropriate headings).  It should have four blocks of three columns of data.  Each block of three columns should include the northing and easting data for one of the four configurations of measurement (Trimble/Garmin, corrected/uncorrected).  The third column in each group should be the planar error from the "known" location of the monument.  It is this error that we will use to run the t-test.

    Run appropriate tests to determine if the Trimble is better than the Garmin (or vice versa!) and to determine if correcting the data makes a difference (for each of the receivers.)  Don't know how to run the test?  Click here for help.

  4. But what about precision?  We will attempt to measure precision by using the standard deviation of the readings in both the northing and the easting data.  (We can't look at the standard deviations of the errors because there is an implicit absolute value in the distance computation.)  Compute the standard deviations for each of your eight sets of readings.  By inspection only, determine whether or not using the corrected signal improves precision.
     

To Hand In

  1. Scientific.  Your lab write-up this week should be in the traditional scientific style.  You should address issues of equipment, methodology, data collection, analysis and conclusions.  In particular, you should include printouts of the spreadsheet showing your data as well as interpretations of what those values mean. 
  2. Non-scientific.  Imagine that you work for a company interested in purchasing one of these two devices.  Each was loaned to you for one day only so that you could perform this lab.  You are to prepare a one-page (max) executive summary detailing which device you recommend and why.  Address the issues that are likely to interest your bosses including both accuracy and precision.

Help Policy

       Help Policy in Effect for This Assignment: Group Project With Limited Collaboration

       In particular, you may discuss the assignment and concepts related to the assignment with the following persons, in addition to an instructor in this course: any GIS instructor and any student enrolled in CS 128/ES 228.

       You may use the following materials produced by other students: materials produced by member of your own group.