Geog 458: Map Sources and Errors

Event 2

 

Evaluating positional accuracy with a GPS receiver

 

 

This event will step you through how to evaluate positional accuracy of test data with a GPS receiver. This event has a lab portion followed by a field portion and followed by a lab portion. Due to limitations of equipment, we will split into three groups. I strongly suggest each group work today and next Wednesday 2:30pm (no lecture on that day) when there’s no obvious time conflict between group members. Each group should:

 

 

1. Identify some features to test on a digital data source

 

It is obvious you should choose test data whose positional accuracy is not better than what you can get from a GPS receiver.

 

The GPS receivers we’re going to use (Garmin eTrex Vista) has 3 meter positional accuracy 95% of the time when it’s WAAS enabled. (google search for the user’s manual)

 

The test data has to be in walking distance of Smith Hall

e.g. City of Seattle, TIGER maps, WAGDA sources….

 

 

2. Select five sample points from the test data

 

See Minnesota handbook for the guideline

 

Label the sample points in the test data

     use selection tool in ArcGIS or mark them in the hardcopy map

 

By the way, are they all qualified for well-defined points?

 

Get the map of test data (where test points are clearly marked) ready

 

The number of sample points should be larger than 20 for statistical statements, but let’s assume you collect 20 with 5.

 

 

3.  Plan out how to visit these points

 

Can you recognize where they are? You may want to display your test data over backdrop data for identifying them better.

 

 

4. Go outside and collect positional values at the test points with a GPS receiver

 

Go outside (a GPS receiver doesn’t work indoors because it can’t receive signals from satellites: the satellite signal can’t penetrate building materials)

 

Turn on a GPS receiver: wait till the receiver tells you “I’m ready”

 

Go to test point 1 (given the test data map), and stop at the point

Hold “Click Stick” (button on the front side: located on top of screen) for two seconds

It will record the positional value at that location

Label the waypoint as t1 or something else you’d like to label (member’s alias name?)

 

Repeat this for other test points

 

*see the manual attached within the GPS receiver unit for detailed instructions

 

 

5. Calculate positional accuracy measure that conforms to NSSDA

 

5-1. Get positional values ready

There are two options for putting data in the GPS receiver to the computer:

1)      If you don’t want to write down (x, y) coordinates one by one, use a software that read the waypoint data (the software “Garmin waypoint manager” is installed at Collab computer, but not in other labs; you also need interface cable)

2)      Well, it’s just 5 points, I would rather put them down in excel file manually

 

5-2. Convert GPS-surveyed positional values into the positional values in the coordinate system of test data

 

5-3. Measure the distance between GPS-points and test points

 

5-4. Obtain RMSE and accuracy measure specified in NSSDA or Minnesota handbook

 

5-5. State accuracy measures

•         Tested __XX.xx__ (meters, feet) horizontal accuracy at 95% confidence level

Of course here we assume the number of sample points are larger than 20

 

Turn in

 

Ø      Title page (should include the names of group members)

Ø      The map of test points (should show if they’re well-defined points and wisely sampled)

Ø      The short description of how you get the distance between test points and reference points (e.g. the conversion of datum and coordinate system, use of measure tool or arctool box for measuring distance, and so on)

Ø      The spreadsheet that shows (x,y) at test points and (x,y) at reference points in the same coordinate system

Ø      NSSDA-compliant statement “tested ____ (meters, feet) …”

 

Due Next Friday 2/24/06

 

Note:

 

If your name is not in the group report, you will automatically lose points (it’s your responsibility to contact any group member to join the group in case you miss the class today on 2/17/06)

 

Feel free to report any concern such as other group members’ negligence – I will use this information to grade them individually

 

No class on 2/22/06 (Meet your group member for completing Event 2)

I will be out of town from this Sunday to 22nd (not in the office)