Assessing Accuracy and Precision of GPS Units

Table of Contents


All GPS units should be tested for accuracy and precision before field use. This can be done by setting up mutliple test sites. Many factors can influence the GPS collars including vegetation density, type of vegetation, being below ground or under water, and being under or around man made structures.

Set up a test site at each habitat type to be tested. Rotate GPS collars between sites and check against locations taken from a handheld GPS, more than one if possible. Check the locations at least once a day. Average all locations taken from each collar at each site and calculate how different the location is from the handheld unit. Ideally you want to test under clear open skies and overcast days in all habitat types, and under multiple satellite configurations.

Posts consist of a 24” stake hammered 6” into the ground with a nail platform to support the collar at 12” in height.

When taking data points determine both accuracy and precision. Precise locations will all be near each other, but do not all have to be near the collar, there could be 10 points that are all 10 feet to the right. Accurate locations will all be near the collar, but do not have to be near each other. Each could be ten feet away from the collar on all sides.

NASA’s Enhanced Vegetation Index can be used to quantify vegetation density.

Collars will be tested at six sites (under limited time drop locations #1 and #3):

  1. Over water (changing water depth and area could falsely influence this location, even day to day)
  2. Open grass
  3. Sparse palm trees
  4. Dense vegetation
  5. In a tree
  6. Near a man-made structure

The collar will remain at each site for one day and be rotated to the next for two rotation. Each collar will be tested at each site on two non-consecutive days.

GPS Collars

When collars arrive remove the magnetic strips and connect the collar to the base station or place it outside in the open to reset the internal clock, the collar will attempt to set the clock every 20 minutes until it is successful. There is a large difference in satellites from California to Florida, for the first location in Florida the collars must be placed in the open or they will time out.

Always double check that long range advertising is enabled on the collars, otherwise getting data will be nearly impossible. Make sure the collars are white-listed on the base station and save a copy of the serial numbers of collars for the same reason.

Before deploying the collars: make sure long range advertising is enabled, and secondary, more frequent, advertising is enabled as well. Ensure the correct schedule is uploaded. Select ACC or Smart GPS. Double check this!


After testing channel 35 is the best, 14 and 34 were also acceptable. Press the lock icon for two seconds to prevent pushing buttons accidentally. Almost any time you are stuck on a weather scan or strange menu button you can get back to radio use by tapping the PTT button. If the battery runs low the radios can be charged with 4 regular or rechargeable AA batteries, but remember to take these out before charging the radio from the radio station!

NOAA Weather Radio- WX BAND

To turn on press the menu button for 5 seconds to enter the weather scan. The radio will scan until it reaches an active channel. To stop the scan press menu briefly, then use the up and down buttons to select a weather channel. To set a weather alert press the menu button twice while on a weather channel and select “ON”. A blinking storm cloud will appear on the screen. If an alert is received in radio mode the radio will beep every 10 seconds and AL and IN icons flash and go to WX BAND. To return to regular radio use, push the PTT button.