Accuracy on electronic maps

I have the Maptech Take a Hike Series (for the Adirondack Park) and I'm concered about it accuracy. For example when I put a route/track right over the Mt Marcy summit I only get an elevation of about 4950 feet (so it's off by about 400 feet). I even put a simple track from one major contour line to another and get nowhere near the elevation change that I should. So now I'm wondering about how accurate all the info is that I get from it (mileage, elevation gain/loss, elevation, etc.). Anyone have experience with this product? Any tips on using it or maybe I'm using it incorrectly? I'm no expert on this thing so I may just be using it incorrectly. Thanks,

Pat

I use the Maptech Terrain Navigator Pro software and find it to be acurate - if you zoom in all the way you can pinpoint the actual summit.

When you download a track from you GPS it may not always appear to be on the trail, but that is due to how often you record track points and the fact that the GPS has some margin of error built in.

Most of these products use what is called a DEM (digital elevation model) The DEM is of fixed distances like a grid of (I can't remember but I believe 100 meters) across the area. Each data point on the DEM has a ground height associated with that particular data point location. Your displayed map is usually a DRG. A digital raster graph. While you are capable of reading the contour lines on it, the computer isn't that smart. What it can do is easily find out the location you are pointing at with the cursor on the DRG and translate that into a location on the ground. Now for the tricky part. Usually, the DEM will not have an elevation for that exact location so, the way it figures out the elevation is to interpolate from the DEM and the nearest 4 elevation data points. The result is a good "guess". Unfortunetly, if the point you are looking at is in the middle of one of these grids and it is also a high point (higher than the 4 surrounding elevation points) then you can have really bad results.

Keith

Some of the digital mapping products use a combination of DEMs and DRGs.

The DRGs (digital raster graphics) are images of USGS paper topos. The 1:24K (or 1:25K) paper topo maps have the following accuracies:
* horizontal: at least 90% within .02 in on map (40 ft on the ground)
* vertical: at least 90% within 1/2 a contour interval

The USGS digitized the paper topos at 300 dpi (dots per inch) or 6.7 ft on the ground to produce the DRGs. The USGS has released these maps to the public for a modest fee (the data is public domain). These maps are available free from websites for many states and are the basis of many (possibly all) consumer digital topo map products.

In the case of NG TOPO!, these DRG maps have been resampled to ~175 dpi to reduce the file size. (Also reduces map resolution.) Don't know what resampling, if any, has been done to the Terrain Navigator maps.

The maps used in consumer GPSes are DLG (digital line graphics) representations of the paper topos. The USGS makes a set of DLG topo maps available on a website free of charge. Presumably these maps are based upon the same data as the paper topos, but I don't know the error specs of the DLGs. (Again one can trade off data-set size for accuracy.)


DEMs (digital elevation models) are available in several resolutions ranging from 90x90 to 30x30 meter horizontal spacing. Don't know what the vertical accuracy is, but a single number representing such a large square obviously misses a lot of detail that might be important to a hiker.


NG TOPO! (and I believe, Terrain Navigator) use DRGs for horizontal detail and DEMs for elevation. Thus it is possible for the contour lines to disagree with altitude readout from the DEM.

Garmin mapping GPSes show an error circle when zoomed in sufficiently on the map display. This error circle includes both the GPS and the map errors. (It is probably a 90-95% probability circle.)

More info at:
http://erg.usgs.gov/isb/pubs/booklets/topo/topo.html
http://erg.usgs.gov/isb/pubs/factsheets/fs17199.html
http://geography.usgs.gov/standards/

Doug

Trail locations are far less reliable on a lot of the digital mapping products. They usually refer back to USGS base maps that frequently show abandoned trails or old trail locations, handy for us bushwackers but dangerous for newbies.

peakbagger said:

Trail locations are far less reliable on a lot of the digital mapping products. They usually refer back to USGS base maps that frequently show abandoned trails or old trail locations, handy for us bushwackers but dangerous for newbies.

Click to expand...

I once used NG Topo! to derermine the waypoint for a Lean-to. Out on the hike the Lean-to was not where my gps said it should be. I figured I had made an error and was puzzling over the discrepancy when one of my hiking companions told me that the LT had been moved. I had a similar experience on a winter hike to Esther from the toll road. Lesson learned: don't trust the map regarding man-made features.

peakbagger said:

Trail locations are far less reliable on a lot of the digital mapping products. They usually refer back to USGS base maps that frequently show abandoned trails or old trail locations, handy for us bushwackers but dangerous for newbies.

Click to expand...

Trails are frequently placed on maps by guesswork and may not be very accurate. The update rate of the USGS topos is pretty slow, so it may take 10s of years for changes to show up.

More recently, some of the trails have been measured using (survey grade) GPS and plotted accurately on maps. (This was done for the Washburn Mt. Washington map.)

Doug

Hikerdad- I use the Terrain Navigator software from Maptech. There is a CD with my version called "enhanced elevation data" that is optional. Once loaded it greatly increases the elevation accuracy of the software. I do not know if there is such a CD for your version but you might want to look into it.

Good luck.

I use mapsource and it is sometimes off to the look however if you zoom in you should find it. also gps is often off depending on the position of the sat. and time of the day etc. but it will get you very close to where you want to go what is 100 ft or so anyway!