+mattb5 Posted February 16, 2003 Posted February 16, 2003 I had posted this in the New England forum but didn't get an anwer to my question. So I figured I'd try it here. Has anyone seen this site yet? New aerial color maps of MA. I saw an article in the paper a few months ago about this, but it wasn't ready yet. Now it is. MA Color Orthos I haven't actually found any practical Geocaching use for it yet, as it doesn't allow coordinate inputs. You just have to muddle around to find landmarks you're familiar with. Anyway, my question is can anyone explain to me what the NAD83 numbers are at the bottom of these maps? It says: NAD83 meters coordinates: 175083.68 890273.65 I tried converting them at Jeeep conversion page but there's nothing that fits that formatting. From what I had read NAD83 is the same as WGS84. But I don't know what to do with these numbers. Quote
+TheGertridgeExplorers Posted February 16, 2003 Posted February 16, 2003 It looks like from the datalayer description that the coords are Massachusetts State planar projection coords. Think of it as a custom UTM. NAD83 is the datum it is referenced to. - Donna G Quote
+mattb5 Posted February 19, 2003 Author Posted February 19, 2003 Wow! WaldenRun had to ask too?! Now I don't feel so stupid. Thanks. I'll have to do some math and see if that works out. Quote
+Waterboy Posted February 22, 2003 Posted February 22, 2003 mattb5 and Warm Fuzzies may be interested in an Army Corps of Engineers program named Corpscon. This is a free download, and may be used to convert state planar coordinates to more recognizable formats. Mattb5, the coordinates you listed as NAD83 meters: 175083.68, 890273.65 will convert to NAD83 N 42º15'45.12444" W071º48'07.17476". A location in the center of the city of Worcester. (I am assuming the first figure of the coordinates is the easting.) I would like to thank Warm Fuzzies for Markwelling us to an interesting NOAA Manual. It is a long manual, but in Section 1.3 titled "SPCS 83 Design" the author discusses problems and inaccuracies of the UTM system. He refers how surveyors and engineers are unhappy with the system, but does not mention how this effects real estate people. In many high population density areas, such as inner cities, small variances in the property lines are important, Worcester is probably one such city. As an example let us look at the coordinates listed by Mattb5 of 175083.68, 890273.65 and call this location A. For location B we will use one 2,000 meters north and 2,000 west of location A or 173083.68, 892273.65. There UTM coordinates would be: Location A: 4682727.88461, 268892.95192 Location B: 4684786.36458, 266951.96761 Calculated distance between A&B using SPC: 2828.42712 Calculated distance between A&B using UTM: 2829.26844 A difference of about 84 centimeters, or about 1 part in 3400. Surveyors measurements show that the state planar coordinate calculations are consistently closer to the actual measured distances than are calculations using UTM. If you use UTM to make calculations please remember that it is most accurate near the center of the zones and least accurate at the borders. Worcester, MA is near the border or near W072º Longitude. The quality of UTM also deteriorates the further the user is away from the equator. Now the important item, whether you use UTM or not THIS WILL NOT EFFECT YOUR GEOCACHING. Quote
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