ArcMap "GPS" Points and Adjustment Lab

Due in about one week.

Objective: The objective of this lab is to provide an introduction to adjusting location data to "known" more accurate points such as might be given by a GPS receiver or (heaven forbid, Google Earth).

You will: (If you want) Setup and use a GPS receiver to record the locations of several points that can be identified in our street data. Use a text file to import those point locations into Arc. Use the locations to assess the street data postional accuracy. Then use Arc's "Spatial Adjustment" Tool to 'move' existing data to fit your measurements.


You will need a GPS receiver (yours or ours), and the allroad3 and cc-streets data sets. (You will remember that these purport to have projection data, and should but do not plot in correct registration with each other.)

You should read the help on "spatial adjsutment" and perhaps the second part of this webpage: .

Part One: GPS points and field notes.

GPS units vary in their controls and capabilities. For this we need to be able to turn on the GPS, set the datum and data format, let it "acquire" satellites and compute its position, and then record a set of desired positions.

The Garmin eTrek Venture's need 2-AA batteries. The power switch is on the right side. Above it is the "page" button. On the front is a "nub" to navigate between and select screen icons (it only seems to go down or left or right and select).

Install batteries if needed. Power on. Page to "Main Menu". Nub to "System" to be sure that the GPS is on (normal) and that WASS is enabled. Nub to "Setup" (press to select), then to "Units. Select the position format (hddd.ddddd) and map datum (WGS 84). Page to the satellites display and wait for the unit to acquire sufficient satellites.

(Warning: low tech, error prone, paper based method ahead!)

On the ground, pick three to five (3, 4 or 5) 'good points' (intersections, maybe corners or ends, but not middles of street segments). It would be good to disperse/distribute these points widely across the island. At each of the places write down the coordinates from the GPS (at least 5 digits to the right of the decimal point, i.e. about 1 meter precision) and a note to yourself and other saying what place the coordinates represent.

Back from the field, type the coordinates into a text file such as you've used before. (Some GPS devices can save data in shaefiles, some don't save it at all.) Get the points into Arc.

Alternatively here are some GPS points from a previous class:

"Intersection", "Latitude", "Longitude"
"Kamehameha Hwy & Lokoea", 21.59435, -158.10316 
"Kahelu & Wikao Pl", 21.59449, -158.10318
"Manoa Stream and Palolo Stream", 21.29080, -157.81473
"Waihi Stream and Waiakekua Streams", 21.3271, -157.8011
and another set extracted from Google Earth
"Intersection", "Longitude", "Latitude"
Paakai St and Paala Lp (huh?), -158.1114758804381, 21.34094325836413
Dole and East West Rd (UH Campus), -157.8152791218156, 21.29618195390237
Kamehameha Hwy and Marconi Rd (Kahuku), -157.9766061498192, 21.6923637009849
Mumba and Midway (Barbers Pt), -158.0826538189028, 21.31513190349621
Kamehameha Hwy and Burroughs Rd (Kahuku), -157.9489957968411, 21.67707013547741
Lunalilo Home Rd and Nawiliwili St (Portlock), -157.7071643484717, 21.27290771903582
N.B. Don't confuse precision with accuracy. And watch the column headers.

Part Two: Text files of point data to maps (you know this)

Idea: It is real handy to be able to symbolize a text file containing coordinates and data on a map.

Set-up: Start with a text file that contains columns (fields) of data. The file name should end with a ".txt" extension. and should contain four columns. Each column should have a heading label, in quotes. Each row should have commas separating the fields. When you cut and past or concatenate files from classmates, be sure to remove any extra heading lines.

Display the points:

Convert text points to a shapefile:

Consider what you see. Do you see discrepencies between your measured coordinates for the places and the datasets' coordinates for these places? What are the sizes and directions of the displacements? Within a shapefile, are they constant or do they seem to vary from place to place?

Part Three: Registering to the data points

Make sure that both the Editor Toolbar and the Spatial Adjustment Toolbar are available, and have the shapefile that you want to adjust and the one that you want to adjust to in the current map.

Turn on editing for the layer that you want to adjust, and set "snapping" for the vertices in those layers

In the Spatial Adjsutment tool...
select tie points, clicking the map point to move and then the point to move it to.
set the adjustment
adjust the data

Now do the data line-up?

Part Four: Deliverable Using the data points

Report the points you used to adjust your datasets.

Report the original discrepencies you found in the data.

Assess the goodness of the adjustment. Do the datasets now line up? Everywhere?

Include 2 maps: one showing the mis-registration of the original data and another showing the degree of registration for the same area after the adjustment.