Posted: 5/1/2009 11:18:57 AM EDT
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I have a few USGS topo maps for my area and they have what I thought was a UTM grid on them. When locating points on various online maps and finding those same cordinates on my topo maps, I was always off by maybe 1000 feet or so.
Recently I ordered a custom topo map from www,mytopo.com. Since the area I hike is right at the edge of USGS topo maps, I had a custom recentered map made. One of the options I had was to put UTM grid lines on the map. When the map arrived, I noticed that the UTM grid lines added (in blue) were not in the same lines as the USGS topo grids (in black) The blue UTM lines added by mytopo.com seem to be correct however, as I can take a corrdinate from an online map, and it corresponds exactly with the coordintes using the added blue UTM lines. My question is.... What are the black grid lines on USGS topo maps? I guess they are military grid lines, which I though had the same positions as UTM lines. Anyone? ETA: The black grids on the USGS topo maps have the same northing and easting as my custom map, however they are off by 1,000 meters in the northing and 100 meters in the easting. |
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UTM is Universal Transverse Mercator, which is usually referecned off an arbitrary origin and measured in northings and eastings in meters.
The black lines are lines of longitude and latitude, referenced off the equator and Greenwich, England. Up to 180 degrees east or west of the prime meridian, and 90 degrees north or south of the equator. I like Lat/Lon, as it's what I've always used and all my cheats use it (for example, 66 feet of horizontal distance at 38N lat is 0.000226 degrees if you're heading east west, but only 0.000181 degrees if you're heading north-south. If you're at 34N lat the same distance is 0.000219 degrees if you're heading east-west. North-south degree-distance conversions remain constant over the entire range of latitudes. Why? Because lines of longitude converge at the poles, meaning that 1 degree of longitude at the equator is a much longer distance than one degree of longitude at 60N). |
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Quoted:
The black lines are lines of longitude and latitude, referenced off the equator and Greenwich, England. Up to 180 degrees east or west of the prime meridian, and 90 degrees north or south of the equator. They are not lat/lon lines, I know that for sure. USGS topo maps use tick marks for the lat/lon. The custom map I have also uses tick marks for lat/lon. The black lines on the USGS maps are definately in reference to a grid system, but they are slightly off from the UTM grid lines. They are 1,000 meters off in the northing and 100 meters off in the easting. I am just not sure why the two grids would be close, but not quite the same. |
| I found my answer by calling the map company. The USGS topo map grids are in reference to the 1927 Datum, and the blue gird lines are in reference to the current 1983 Datum. (They can set your grid lines to either datum I believe, I just chose the current datum when I ordered my map.) |
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Quoted:
I found my answer by calling the map company. The USGS topo map grids are in reference to the 1927 Datum, and the blue gird lines are in reference to the current 1983 Datum. (They can set your grid lines to either datum I believe, I just chose the current datum when I ordered my map.) yep topo maps are not updated often depending on where on the country you are it may vary in how far it is off |
| I have encountered lots of maps still used for training in the military that are NAD27 map datum. The current datum is WGS 84, there is a significant difference in them if you are land navigating or calling in airstrikes as I did in the mil. Make sure your GPS is set to the right datum, or use lat longs. |