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GPS

There is a great variety of GPS available, and most will do a great job at getting your position.

These are websites where you can get comparative prices: For reviews, you should just Google "handheld GPS" and read a few sites. This varies by the month. After you find out what you want, for finding compatative prices, you can try http://www.nextag.com and request handheld gps. That lists hundreds of them from dozens of shops with the lowest prices. Also try http://amazon.com.

Accuracy

The accuracy of most cheap hand-held systems in sufficient for most uses. Of course, the producers like to sell you more expensive models, so I expect that they list higher accuracies for their more expensive models than for their cheapest models.

In order to get the highest accuracy possible from your equipment consider this:

Wake / Warm Up - Turn on the GPS unit at least 5 to 10 minutes before you plan to use the unit.

Keep it on - Leave the unit on so it will continue to collect data and updating the ephemeris data which needs to be updated about every hour. Avoid putting the unit inside your jacket or pack.

Differential Correction - Use some form of differential corrections (DGPS) such as the Wide Area Augmentation System (WAAS) in the USA. Most differential corrections will allow accuracy of 3-5 meters and WAAS will allow accuracy to 1 to 3 meters. See my GPS Accuracy Testing page for test results of a WAAS unit vs. non-WAAS unit. Keep in mind that on some GPS units that have WAAS still must be turned on or enabled.

Average - Use "Averaged" waypoint to locate the point. Most units will start Averaging once the unit sits still for a few seconds or there is a way to tell the unit you want to start averaging. Allow the unit to average for 30 to 60 seconds.

Record the Data - Use the unit to store the point vs. writing it down. This reduces the chance of a transcription error.

Know the Units - If you do record it by writing it down, make sure you record down to a unit that will give you the precision you require. To get down to around 5 feet or so you will need to record to:

	00.00001° (Decimal Degrees)
	00° 00.001' (Degrees, and decimal minutes)
	00° 00' 00.01" (Degrees, minutes, seconds)

Avoid Blocks - Make sure the unit is not blocked from the satellites. If you are beside a building or cliff (especially if it is to the south) it will partially block the signal from the satellites. If you are under thick tree cover, you will have reduced coverage. You may need an external antenna. Some of these will increase the signal in areas of poor reception.

Avoid Bouncing - Avoid getting a multi-path error which is caused by being close to an object that is bouncing the signal. Again, avoid standing close to buildings or other reflecting objects. If bald, wear a cap and cover other gear with hard surfaces.

Data Quality - Most GPS units will provide you with an indication of the quality of data. For example, Estimated Position Error (EPE) or Position dilution of precision (PDOP). The smaller these numbers are the better (for PDOP less than 6).

Plan - Due to the way satellites orbit the Earth, there are times when coverage is good and poor. You can download Mission Planning Software from Trimble or Thales . Once installed, you will need to download the most current Ephemeris Files aka Almanac (Click here for Trimble and here for Thales.