Nick_T Posted March 27, 2004 Share Posted March 27, 2004 Keep seeing all these posts with thisx60= seconds and such and can't figure out why more folks aren't using UTMs where 4+2=6. Is there some sort of advantage of using Lat/Long that I'm missing? The UTM maps and system are really easy to understand and learn, expecially for someone that is new to GPS/Map applications. What's the deal with this? Nick T Quote Link to comment
+Renegade Knight Posted March 27, 2004 Share Posted March 27, 2004 LL is the same coordinate system world wide. UTM works great when you are in the UTM zone the coordinates came from. Cross zones though and it gets more complicated. Still for solving math caches UTM can't be beat. Quote Link to comment
+Prime Suspect Posted March 27, 2004 Share Posted March 27, 2004 Keep seeing all these posts with thisx60= seconds and such and can't figure out why more folks aren't using UTMs where 4+2=6. Is there some sort of advantage of using Lat/Long that I'm missing? The UTM maps and system are really easy to understand and learn, expecially for someone that is new to GPS/Map applications. What's the deal with this? Nick T As RK said, UTM is only useful for a relatively small area. Beyond that, the inherent inaccuracies built into the system (caused by not considering the curvature of the earth) start to rear their ugly head. That's when you find out that 4+2 doesn't equal 6, and you need to find out how to do a Great Circle calculation. Quote Link to comment
Nick_T Posted March 28, 2004 Author Share Posted March 28, 2004 OK, I didn't know this. Where does the inaccuracies start showing up? Lives depend on the accuracy of the information I give them from my unit, I don't only use GPS for play. I'm using this in a relatively small area, one state, and it has been flawless so far. Nick T Quote Link to comment
+tozainamboku Posted March 28, 2004 Share Posted March 28, 2004 UTM is a projection of the curved earth onto a flat plane (your map). Think of taking a thin segment of the curved suface of the earth and flattening it out. You will get some distortion at the edges of the segment. When the segment is very thin the distortion won't be very big. For UTM the segment is six degrees, so the distortion isn't very big. And the distortion is greatest at the edges of the UTM zone. For the most part the distortion will be less than the accuracy of your GPS. Quote Link to comment
+tozainamboku Posted March 28, 2004 Share Posted March 28, 2004 RK and PS are correct in that if you are trying to determine distance between two points on earth that are in different UTM zones you will probably have to convert the UTM coords to lat/long and do a great circle calculation. It isn't meaningful to compare UTM coordinates from different zones. Quote Link to comment
Tahosa and Sons Posted March 28, 2004 Share Posted March 28, 2004 OK, I didn't know this. Where does the inaccuracies start showing up? Lives depend on the accuracy of the information I give them from my unit, I don't only use GPS for play. I'm using this in a relatively small area, one state, and it has been flawless so far. Nick T You've hit the nail on the head. The inaccuracies in a single zone are caused by different datums. What I mean is this, say a lost hiker with an old map reads off the utm's and doesn't say what the datum is. And the SR people are using the computer generated software on WGS94. So now we have a problem people, does it mean to search in 2 places or hope for the best. And I wish I really knew the answer so everybody could be on the same page. Quote Link to comment
+tozainamboku Posted March 28, 2004 Share Posted March 28, 2004 Using different datums is a problem whether you are using lat/long or UTM. Since the earth is not exactly a sphere (perfectly round), a datum is a set of parameters that define the shape of earth. The different datums give slightly different shapes and therefore different latitude and longitude for the same location. When you are giving someone your coordinates it is important to specify which datum you are using. T & S point is that many topographic maps use the NAD-27 datum. If your GPS is set to the WGS94 datum it won't show the same position as your map regardless if you are using UTM or lat/long. You can't change the datum of your map but most GPS units will let you change the datum they use for displaying location. Quote Link to comment
+Waterboy Posted March 29, 2004 Share Posted March 29, 2004 OK, I didn't know this. Where does the inaccuracies start showing up? Lives depend on the accuracy of the information I give them from my unit, I don't only use GPS for play. I'm using this in a relatively small area, one state, and it has been flawless so far. Nick T Taking a quick look at the longitudes of caches you found recently I see numbers between W083º30 and W083º55. There is a UTM zone change at W084º00. You are in Zone 17, west of your is zone 16. This is when your math becomes complicated. Similar zone changes will occur at W072º00, W078º00, W090º00 and W096º00 or any other multiple of six degree longitude away from W084º00. An additional note is that UTM has problems in polar regions. I do not believe this has effected any geocachers yet. Quote Link to comment
+Prime Suspect Posted March 30, 2004 Share Posted March 30, 2004 An additional note is that UTM has problems in polar regions. I do not believe this has effected any geocachers yet. That's why Santa only uses WGS-84. Quote Link to comment
+hatchet hutch Posted March 30, 2004 Share Posted March 30, 2004 I used Lat/Long cause I started to use UTMs with Arcview. Downloading the corrds from the web site got to be --- troublesome. On the ground it took me three times before I relized the system was not convertiing. Using lat/long went right to the first cache, and the rest is history. Hutch Quote Link to comment
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