A. Download the Lesson Data
My story:
If you recall, one of the layers that the State College Borough Water Authority identified on their GIS wish list was fire hydrants. I decided to acquire hydrant data for this activity even though that type of data may call for sub-meter accuracy, something my recreational grade Garmin eTrex receiver will not give me. I noticed that there are some new developments in Pine Grove Mills that aren't included in the CAD data yet. Acquiring features in new developments is a great use for GPS.
NOTE -- If you are going to be collecting data in neighborhoods or in some place where your activities may look suspicious, it is a good idea to let the police department know where you will be and what you are doing in case someone calls. In these days of heightened security awareness, I suggest that you stay away from obvious military or government facilities.
Registered Students download from ANGEL the Lesson 7 data (lesson7files.zip) to a new folder (e.g., C:\MGIS\GEOG488\Lesson7). This .zip file is about 73 mb.
B. Load the Basemap and GPS Data
We will be overlaying the GPS data I acquired with some of the county basemap data you've seen in previous lessons.
- Add the streets layer first. The data frame will assume the projection of that layer.
- Add the digital orthophoto clip_9230a.tif.
- Add the PGM_hydrants layer. These are the hydrants I acquired with the GPS. My receiver units are: Position Format: degrees, minutes, seconds; Map Datum: WGS 84.
- Rather than adding attributes directly into the GPS unit, I took notes in the field, being sure to include the waypoint number with each hydrant number. I personally always keep a data log incase of subsequent problems with the unit. Add the hydrants_lookup.dbf table into ArcMap.
- Join the hydrants_lookup.dbf table to the PGM_hydrants layer using the waypoint number for the join.
- In the next section, you will be adding fields and populating them. Export this layer to make the join permanent. Give the layer a new name such as PGM_hyd_ids.
- If you zoom around the ortho, you can see that one hydrant I acquired (z2-10-05) is in a development that was put in since the aerials were flown.
C. Add X and Y Coordinates
You'll notice that the X and Y coordinates are not included in the table.
- Open the Attributes of PGM_hyd_ids table.
- Add fields to hold the coordinates, e.g. X_coord and Y_coord. Be sure to specify parameters that will be able to hold the values, e.g. type: double, precision: 10, scale:6
- Right-click the field heading for the X field and click Calculate Geometry.
- Calculate the X Coordinate of a Point
- Right-click the field heading for the Y field and click Calculate Geometry.
- Calculate the Y Coordinate of a Point
D. Compare GPS Coordinates to a Known Location
- Add the benchmarks layer. These are benchmarks from the National Geodetic Survey site [http://www.ngs.noaa.gov/].
- Add the digital orthophoto clip_9243c.tif.
- Add the psu_ngs layer. These are GPS points I collected for one of the NGS benchmarks on the PSU campus.
- Measure the distances between the collected points and the benchmark with known coordinates. The 16 locations I acquired are off between 2 feet and 15 feet - not bad!
Now, you will be given some instructions on how to acquire these benchmarks from the NGS site to test the accuracy of your receiver.
E. GPS a National Geodetic Survey Control Station
- Go to the National Geodetic Survey site [http://www.ngs.noaa.gov/].
- Click the datasheets link.
- Click the Shapefiles button.
- You can pick from any of the Retrieval Methods, but these instructions will be for a Radial Search.
- Enter a Center Latitude and Center Longitude. These can be acquired with your GPS receiver.
- Enter a Radius.
- Keep the defaults for Data Type, Stability, and Compression Options.
- Enter a File Prefix and click Submit.
- A list of stations should appear. Highlight the stations you want to download or click Select All and click Get Shapefile.
- Save the file to your Lesson 7_8 directory. Unzip the file and add the shapefile to ArcMap.
- Now, you should now GPS these stations to see how accurate your GPS data are.
F. Optional Go Further into GPS and Location
Here is a look at geodetics and GPS
That's it for Part I!
You have just completed Part I of this module, which involved looking at acquired GPS data and adding X and Y coordinates. In Part II, you will load GPS data you've acquired.