Saturday, July 26, 2008

GPS Essentials

And we continue on with fieldwork-related posts. . .This one will probably be old hat for some of the more experienced readers out there, but I figure it doesn't hurt to lay it out for those who are curious, or those who are perhaps new to the fold. I'm talking about taking GPS coordinates!

In my last post, I discussed the importance of taking the right kinds of field photographs for documentation. A second important facet of good field documentation is GPS coordinates. Just like with photos, not all GPS coordinates are created equal.

When taking coordinates, whether they're for your own records or in order to pass them on to a colleague, context is everything. When I take a GPS coordinate in the field, I note several things:

-Coordinate system. What system are you using? Lat/long? WGS coordinates? It's self-explanatory, but at least be aware of the coordinate notation.

-Time. I always write down the time of the GPS reading. I suspect that time doesn't matter horribly in most cases, but I like to have it in the field notes "just in case."

-Accuracy. GPS units will often give you a readout of the estimated accuracy of your current reading (usually between 3 and 5 m, on the best of days, for my little Garmin). I always write this down; it's especially important to know if your coordinates might be drastically off (due to weather, topography, trees, etc.).

-Machinery. What GPS machine are using to take the readings? This is useful for real GPS geeks, who may want to know the type of firmware, software, conversion algorithms, etc.

-Datum. This is really, really critical, and a piece of information that is often neglected. NAD27? WGS84? It's all cryptic letters and numbers, but noting the datum is crucial for a person to be able to relocate your site or map it properly. Each of these datums are based on a different ellipsoid model for the surface of the earth. There isn't space to talk about the logic and meaning behind the different systems (see a decent review here), but suffice it to say that the difference between coordinates in two different datums may be tens of meters. Believe me, it's frustrating to get a set of coordinates without knowing the datum - it can be nearly as bad as not having any coordinates at all!

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