What gives?

Same numbers result in different location

Photo: George Beckwith

If you’re on hot bird action, and you send the location to your buddy, he’d better enter the numbers using the same datum you’re using.

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Dear Capt. Paul:
I have some coordinates directly from a buddy’s GPS, but they are not bringing me to the same location as his GPS?
Are there multiple ways to measure/input coordinates? What are we missing here?
Thanks.

Capt. Paul’s response:

n order for his position coordinates to indicate the same position in your GPS unit, they must be entered using the same settings that he used to mark or enter those coordinates.

Check the following configuration settings in his and your unit:

The LOCATION, or sometimes called the POSITION FORMAT. Most GPS receivers now use a format of degrees, minutes and thousandths of minutes. It will be shown as DDD,MM.mmm.

The DATUM settings of both units. They must both be the same in order for the value of the actual positions to be the same. Common settings are NAD 29, NAD 63, and WGS 84.

The difference between NAD 20 and WGS 84, two of the more common in our area, is about 160+ feet. Using other datum settings could result in a difference of several miles.

Check if your and his GPS both have the new WAAS differential ENABLED. WAAS allows modern units to discern a position to within 3 meters of accuracy. Without it, a consumer GPS unit can only display a position to about 15 meters.

If you’d like, email me (captainpaul@charter.net) one of the positions, and I will plot it for you using the two most common settings.