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Scott&Kristi

Different Datums?

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Most (all?) of the geocaching website uses coordinates based on the WGS84 datum. I notice that most of these markers are identified by NAD83 coordinates.

 

I did a few web searches and it appears that these two datums are almost (but not quite) identical. One site claimed differences could be "as much as" 1.5 meters.

 

I'm no expert on these matters, but it appears that dont need to be concerned about the different datums.

 

Can someone smarter than me confirm or refute this conclusion?

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As it has been said, NAD83 is "good enough for government work" - Almost identical to WGS84.

 

Jeremy

 

'

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A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey. Early maps and surveys were carried out by teams of surveyors on the ground using transited and distance measuring "chains". Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders either. The "known points" and their positions are the information that the map datum is based. As space based surveying came into use, a standardized datum based on the center of the earth was developed.

 

Every map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any precision will also list the datum used on the map.

 

The Global Positioning System uses an earth centered datum called the World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. For all practical purposes there is no difference between WGS 84 and NAD 83.

 

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NAD 27. (Some GPS units subdivide this datum into several datums spread over the continent. In the Continental United States use NAD27 CONUS.)

 

In the Continental United States the difference between WGS 84 and NAD 27 can be as much as 200 meters.

 

You should always set your GPS unit's datum to match the datum of the map you are using.

 

On a USGS topographic map the datum information is in the fine print at the bottom left of the map. The datum will always be NAD 27. There may be information on how many meters to shift a position to convert it to NAD 83. Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of the map gives a visual indication of the difference between the two datums.

 

FYI,

 

Mtn_Bkng_Dave icon_biggrin.gif

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A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey. Early maps and surveys were carried out by teams of surveyors on the ground using transited and distance measuring "chains". Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders either. The "known points" and their positions are the information that the map datum is based. As space based surveying came into use, a standardized datum based on the center of the earth was developed.

 

Every map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any precision will also list the datum used on the map.

 

The Global Positioning System uses an earth centered datum called the World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. For all practical purposes there is no difference between WGS 84 and NAD 83.

 

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NAD 27. (Some GPS units subdivide this datum into several datums spread over the continent. In the Continental United States use NAD27 CONUS.)

 

In the Continental United States the difference between WGS 84 and NAD 27 can be as much as 200 meters.

 

You should always set your GPS unit's datum to match the datum of the map you are using.

 

On a USGS topographic map the datum information is in the fine print at the bottom left of the map. The datum will always be NAD 27. There may be information on how many meters to shift a position to convert it to NAD 83. Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of the map gives a visual indication of the difference between the two datums.

 

FYI,

 

Mtn_Bkng_Dave icon_biggrin.gif

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I went out to try one of these this afternoon, and the coordinates were WAY off. Like about 300 feet! Luckily, the description of the location led me right to the benchmark. I can't believe that the difference between WGS84 and NAD83 is that big, so as a test, I tried calculating the difference between NAD27 and NAD83 for that location. Came out to 94 meters, pretty close to what I observed.

 

Are you absolutely sure that the coordinates for all the benchmarks are NAD83???

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The errors are almost certainly the +/-6 seconds scaling accuracy from the maps. The best way to be sure is to find a horizontal control station and see how close the coords are. The horizontal marks are not scaled off maps, but rather are surveyed to within inches. These will probably be very close, at least within the error range of your GPS.

 

rdw

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That's very interesting about the horizontal control stations! I'll have to check out the one nearest my house.

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I added the control info to the benchmark pages. You can now see the horizontal and vertical accuracy, and a link to the bottom of the page with more info.

 

Jeremy

 

Jeremy Irish

Groundspeak - The Language of Location

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