Accuracy of Elevation Data Versus Roads/Streets/Lakes

[trailrunners]

I just recently picked up Topo USA 5.0. I had version 2.0, but was disapointed with the way the maps looked and some of the importing of data for tracks, trails, etc. I'm a bit happier with version 5.0, the maps do look better and a bit more accurate, but I'm still a bit disappointed with what appears to be the overall accuracy of the maps.

What I'd like to determine is whether the elevation data (location of the topo lines) can be counted on as being more accurate than the placement of streets, roads and water sources. Sometimes I see topo lines in a lake or water source (happens a lot actually). Also, when I recently imported a track from my GPS which appeared to be very accurate in relation to streets and other feutures on an aerial photograph, did not look correct when imported into Topo USA. The tracks went over the top of streets when I know for sure that I never crossed the street. I'm assuming that the placement of the streets was sloppy and not in the correct location. That being said, if I can at least count on the elevation data being correct then it isn't quite as bad. Can anyone confirm the accuracy of Topo USA's elevation data?

Overall, I'd have to say I'm disappointed with TOPO USA 5.0 and I'm happy I didn't spend too much money on it. It will be helpful and fun to use in some situations, but won't be a dependable tool for any kind of accurate mapping purposes. Guess I'll have to spend a whole lot more money for that kind of mapping program. Any help greatly appreciated!

Thanks!

[Sanaghan]

At the very top of the forum you will find a stickie titled:

"Sticky: Why is Topo horizontally/vertically inaccurate?"
It takes you to a detailed explanation, located here:
http://forum.delorme.com/viewtopic.php?t=11777

Simple version: The USGS provided Digital Elevation Map underlying Topo is more accurate than your GPS. Your GPS has a choice in accuracy: X-Y (lat,lon), or Z plane (altitude). There's a sacrifice in Z to make X-Y more accurate, as that's what most people worry about when traveling.

This is similar in concept to your computer speeds. You have an overall max transfer speed. Download is (for home use) faster than upload. Why? Do you click and download more information, or do you sit at home and push out / upload more information?

Read the link for more detailed information. Topo 5 also came out in 2003 IIRC, the information (including the DEM) was updated in Topo 6.

In general, the roads are best at major roads and highways that are "pinning points." Accuracy is not as perfect between them, but not so bad I ever got lost. I see about 300 feet typical worst-case variation between these pinning-points.

Note that GPS variations affect accuracy, and you will find away from main cities/highways, NavTeq (MS S&T, google, etc) accurcy goes down as well. They cherry pick.

• Horizontal Road/POI Inaccuracies: Maximize your screen before clicking the link (it's a png and sizes to your screen size). 121 meters off over the same day. It will explain some of the inaccuracies. You might also be in a region that needs to be submitted for a correction as the roads are not located correctly and need to be shifted over.

[trailrunners]

Your explanation was very helpful - thanks a lot for the reply! I will read the sticky you mentioned more closely.

Based on the aerial view I looked at (which should be very accurate, because it shows clearly the actual streets and points of interest that are there) the track from my GPS looks very accurately placed. So, from your explanation, basically what I thought is correct. The street is probably not placed accurately, it is not a major road or highway, but most likely, I can count on the elevation data (topo lines) to be accurate. That's a good thing.

I wasn't really asking about the accuracy of my GPS track - I know that's likely to be off a little. I was just questioning the relative accuracy of the POI/street data and the elevation data. My GPS track was just providing an example of the likely inaccuracy of the street I was tracking near.

It's good to know that the elevation data is accurate though. I do like the Topo USA product - it's the best there is out there right now for complete USA coverage, but it would be nice to have a little more accuracy for all the streets and POIs.

By the way, is the Topo USA 6.0 product a significant improvement over Topo USA 5.0? Are the elevation data or streets/POIs any more accurate?

Thanks!

[Sanaghan]

All the streets they updated are more accurate.

This post: http://forum.delorme.com/viewtopic.php?t=8489
has a great history on DeLorme and their map data, why they switched to internal controls, and why it's getting better.

Note that I've seen tracks from rural Arizona and rural Alaska now that you can save as GPX, then import into NavTeq data (Garmin / Streets & Trips / Google all take GPX). In many areas, DeLorme has surpassed them. Unfortunately, DeLorme doesn't have a huge fleet of GPS enabled vans and while their data is good in cities, it's not always dead on. But I've never gotten lost.

One trick other map makers use is "snap-to" roads. This has caused me problems with my garmin when the roads have changed and I snap to the wrong ones, or when I travel parallel to a route for a while and it thinks I'm on a different road. DeLorme shows you your exact position (even if the roads aren't perfect) and trusts you to use your brain.

The DeLorme tools, data layering, and ability to add new roads and trails more than make up for that to me.

If you get an opportunity, Topo 6 was a nice upgrade.