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 anneminix 11-25-2003 06:51 AM

utm vs lat/long

i have yet to see the use of universal transverse mercator (utm) vs lat/lon for marine navigation. since i use utm all the time on land and find it so much easier/faster that lat/lon i am wondering why its not used for marine purposes..tks GBA

 pblais 11-25-2003 02:07 PM

utm vs lat/long

UTM and Lat/Long are almost interchangeable. It''s just the unit of measure.

What you find is that not all charts come in both units of measure. Sometimes they come only one way or another. The math by hand is a tad easier with the metric version and I can see why you would prefer it.

In the end if you feel more comfortable one way or the other then you''ll probably make fewer mistakes going with what you like.

FWIW, the conversion software to go between the two systems is far more accurate than the ability to make the chart.

 mcain 11-26-2003 06:28 AM

utm vs lat/long

Lat Long is a universal measurement of earth coordinates, in theory independent of map projection. A map projection (e.g. UTM, Albers Equal Area, etc) is a method of mapping the ellipsoid onto a flat surface. THere are probably 20 map projections in common use. Every mapping introduces distortions. Different kinds of distortions are present in the different projections. But all representations of the surface of the ellipsoid earth onto a flat surface will be distorted somewhere. Some distort angles, some areas, some distances. We have all seen the massive size of Greenland in grade school representations of the earth on a Mercator projection. For UTM, within a zone, the Northing and Easting coordinates are X-Y coordinates and can be used to make distance measurements, area measurements, and even direction measurements--all with some measure of inaccuracy and distortion. Going between zones is worse.
Lat Long is ideal for taking sights, etc (with a sextant). If you know very well what you are doing, the inaccuracies in a standard map projection are intended to be insignificant for normal use of the map.
Mathematical transformation between lat long and any map projection typically requires only the two axes of the earth as input--yes the two axes are different. Also, since the earth size has been measured many times, with different answers each time, you have to select the model of the earth ellipsoid you wish to use. Then the math transformation is trivial.
Normally N,E,Z are easier to scale off large scale topo maps, and if you have a handy computer, then transform to lat long or any other projection of interest.
All this said, I have seen the projection coordinates more often used in land mapping or map transformations for spacecraft imagery or high-altitude plane photo mapping, than for anything marine. If I were doing it, I''d stick with the standard marine stuff.

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