Idaho Airships, Inc.

For my friends mired in the Google Maps Flash API or georegistering their aerial images, here’s a good resource to help in customizing placement of Markers, Info Windows, Polygon Vertices and other Lat/Lng features.

When dealing with Lat/Lng Objects, which use the DDD.dddddd syntax, make your placement or offset calculations with this calculator.

I “rough in” my calculations with the following “Rules of Thumb”:

For Latitude, the first decimal place increments about 7 miles for each point (i.e. 43.1 to 43.2 is about 7 miles). The second decimal place is one-tenth of that distance, or about .7 miles. The third decimal place is one tenth of that distance, or ~.07 miles (~350′), and the fourth, ~.007 miles (~35′). There’s very little necessity to graduate your specifications beyond +/- 17′ (an ~35′ range of error), so you may wish to standardize your Lat/Lng to 4 decimal places.

For Longitude (yes, it varies with Latitude, so you’ll need to calculate the actual increment for each location), the first decimal place increments about 5 Miles around Boise’s Latitude, the second about half a mile (~2600′), the third about 1/20 mile (~250′), and the fourth about 1/200 of a mile (~25′). If you are using a higher grade handheld GPS unit such as the Garmin HCx Vista you will find that you regularly exceed 15′ in accuracy so there may be some call for graduating your Lng Point to the fifth decimal.

Always keep in mind that if you are using a GPS sampling of say, 2 seconds at an accuracy of 15′, but you are traveling at 150 miles per hour there may be some “slop” in the registration of your GPS collection and the correlation to your images (which may be snapped anywhere in between your sampling frequency). Thus, your images-unless registered exactly to your GPS track log, will not enjoy the same accuracy as your GPS device is offering. The error will obviously mitigate as your speed slows.

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