Map accuracy

[buddy]

About two years ago I purchased a GPS because I became convinced that some of the USGS quadrants were more accurate in depicting topagraphy than others. I was hiking in western MA, very carefully consulting my map and compass when things on the ground did not appear to topographicaly correlate at all with what I saw on the map. I was pretty sure of my location because we found a state forest corner boundary marker that was depicted on the map. However the contours made no sense. With a little internet research and asking around I found that not all quadrants are created equal. National and state parks, areas of development such as dams and highways have of course been accurately surveyed, but some areas are given less attention. Has anybody else found inaccuracies in Topo's, after years of taking USGS maps as the word of god I'm feeling let down. Please convince me it isn't so!!! I've always gotten to where I wanted to go, but GPS tracks have shown me I didn't always know where I was, esp. in areas where land marks are unremarkable.

 

[dms]

The most amazing inaccuracy on a USGS map I have ever seen is the non existence of a mountain shown as W. Tumbledown in ME. The peak was on the original 3k list until the first completers of that list "discovered" it wasn't there. The newest version of that quad corrects the error. See http://docs.unh.edu/me/spen32nw.jpg for the original map.

 

[mavs00]

Yes, many of the maps put out by the USGS have some real discrepancies to them. A similar question came up in discussion on my forum as it related to the ADK HH list. and Tony Goodwin (the ADK guru that litterally wrote the book) provided some very enlightning answers.

It's in the ADK 100 highest section of my site, which requires registration to view (sorry ), but if you a member, you can view it -HERE-. If your not a member, in-sum an substance, ummmm....... it's a govermental agency .

 

[lumberzac]

Not all USGS maps are the same. In particular pay close attention to contour intervals. In the Adirondacks I have seen 3 different contour intervals (there are probably more) used, 10’, 6m, and 10m. This get especially confusing if you are hiking in any area covered by two maps and they using different contours.

I also know where there are some ledges that don’t show on any USGS map I have seen, but this may have more to do with the fact that they are only 20 to 30 feet high which isn’t a significant elevation change.

 

[DougPaul]

The specs for 1:24K USGS topo (paper) map accuracies are:
* horizontal: at least 90% of the points tested must be within .02 in on map (40 ft on the ground)
* vertical: at least 90% of the points tested must be within 1/2 a contour interval

Ref: http://erg.usgs.gov/isb/pubs/booklets/topo/topo.html

Obviously, these specs allow for 10% of the points tested to be outside of the limits given above, but the spec gives no limit on how inaccurate these points might be...

Doug

 

[ChrisB]

Incorrect GPS datum in use?

Very often if your GPS and the map disagree by a significant amount (30 meters or more of ground distance), the GPS is using a different datum than the map is based on.

Be sure that the datum selected in the GPS unit matches the datum printed on the physical map.

For the Whites, there are two or three different datums you'll encounter on topos.

cb

 

[Nessmuk]

Has anybody else found inaccuracies in Topo's, after years of taking USGS maps as the word of god I'm feeling let down.

True errors, while rare, do exist and I have found a few mostly minor ones on Topos. Unfortunately I often rely on subtle map features and such errors are endlessly frustrating. All the more reason to be sure you use multiple clues and fit everything together, including the rare possibility of map errors.

The map errors you most have to worry about are the much more common falsely perceived errors.

 

[RoySwkr]

The specs for 1:24K USGS topo (paper) map accuracies are:

If a [newer] USGS map complies with the above, it should have printed on it "This map complies with National Accuracy Map Standards" or somesuch. If it doesn't, it may be printed anyway as a "provisional" and won't say that because there's no money to fix it.

Over a century ago, the original Bell system decided to supply their own power to phone lines as otherwise reliability of the phone system would be the same as the worst electric company in the nation. Similarly, the overall accuracy of a map can be ruined by sloppy work in just one area.

 

[buddy]

Good Tip

Thank you ChrisB for the excellent point about making sure the Datum on the map matches the GPS setting, I'll have to pay more attention to that detail. Nessmunk's point about perceived map errors is well taken, it's kind of like not believing your compass. Magnetic abborations do exist, esp. in my mind, but they're not that common. Buddy

 

[DougPaul]

Be sure that the datum selected in the GPS unit matches the datum printed on the physical map.

The difference in location between datums NAD27 and WGS84 is about 230 meters (750 ft) in the Crawford Notch area. Enough to be rather significant...

Doug

 

[Mark Schaefer]

Overall I have found the general contour topography of the USGS quads to be fairly adequate, especially in well-travelled mountainous areas.

Some place names have been incorrectly shown. A couple of Catskill examples:

• the "South Mountain" just north of the Kaaterskill Clove in the Catskills (Kaaterskill quad). It was:
  - Correctly shown on the 15 minute 1903 quad.
  - Incorrectly placed south of Kaaterskill Clove (on a summit east of Kaaterskill High Peak) on the 7.5 minute 1946 quad.
  - Correctly placed again on the 1980 reprint of the 7.5 minute quad (as it currently appears on topozone).
• the Pratt Rocks (a mini vanity "Mount Rushmore") have always been shown on the wrong side of its hill. They are located at this red mark.

Zac is correct concerning some cliffs/ledges not being shown very accurately, such as the Kaaterskill Falls. The upper falls is over 180' high. Other than a slight grazing of the rock wall about 1/3 of the way down, the waterfall is a sheer vertical drop. However, every 20' contour line is shown on the topo map. The visibility of each contour line is a bit clearer on the actual paper map, than on a computer screen.

I also know numerous 100' and higher vertical cliffs in the Shawangunks which are understated (by the number of converged contour lines) on the topo maps. Some examples are in the Gertrude's Nose and Millbrook Mountain cliff areas south of Lake Minnewaska.

 

[Raymond]

I've read that Panther Mountain in the Adirondacks was originally called Henderson Mountain, and the present-day Henderson was originally Panther, but some mapmaker switched the names and they've stuck ever since.

 

[John Graham]

The USGS has been struggling with a meager budget for decades, it has no political constituency and "small" government has been in voque lately. The USGS contrasts rather markedly with the Ordinance Survey in the United Kingdom. There are certain functions that are vital, and can only be performed by the government, such as nation wide topographic surveys and hurricane preparedness and response.

 

[John H Swanson]

In my travels I have seen two big errors.

The issue of West Tumbledown / Leroy Mountain in Maine as mentioned by Dennis.

A convergence of two streams 200 ft different (lower if I remember correctly) located in the Pemmi going east from N Hitchcock to the Cedar Brook Trail.

 

[canoehead]

maps

i have found when buying topo maps you need to make sure that they are the latest upto date verson. not the fifteen year old ones hanging out on the dusty shelf at the local general store. check the date on the map. i just came back from Ecuador, i was climbing in cotopaxi nat'l park, and then down in the rain forest and the most recent map we could find were from 1969 yes there were many changes but i found if you stick to good basic skills you'll manage.