Author Topic: Topo lines and 3D don't overlay correctly (Read 5702 times)

kseddy · « **on:** September 27, 2005, 08:32:45 AM »

My area is relatively flat with changes in elevations under 100 ft. When I make the topo into a 3D map the elevations do not line up with the topo lines, i.e., some topo lines go up/down in elevation. What's up with this? Help!

Krein · « **Reply #1 on:** September 27, 2005, 08:46:39 AM »

Can you post a picture or provide some coordinates for where you are?

Thanks,.

kseddy · « **Reply #2 on:** September 28, 2005, 06:14:23 AM »

My coordinates are 43.80420 N, 84.17678 W.
Notice in #D mode the topo lines are at different elevations. Low-lying areas area some times shown as hills and visa versa.

I'm not sure how to post a picture.
Thanks for any help you can give.

P.S. Aside from this issue, TF is awesome! I'm looking forward to purchasing the full version if the topo overlays work.

keddy@newcenturyeng.com

Krein · « **Reply #3 on:** September 28, 2005, 07:26:08 PM »

Hi,

OK, as I thought, you're using 3D in a relatively flat area.

Go to 3d settings and change the "Data Source" setting to Always Download (US only). This gives higher resolution DEM data which matches better to the TOPO lines.

The "World Only" setting is really only for larger models (less detail). It has some variations that are definitely not represented in the actual terrain.

Glad you enjoy the program.

kseddy · « **Reply #4 on:** September 29, 2005, 05:30:54 AM »

I already have the 3D setting to "Always Download (US only)" and I set he vertical adjustment to 8. The topo lines still don't match the 3D terrain. It looks like the blended pictures are rotated about 5 degrees and stretched a little. Which is more representative of the actual terrain, the 3D model or the topo lines?

Also, why does the 3D image have more topo gradients than the topo map. Are the 3D gradients actual? Down to .1 meters?

Krein · « **Reply #5 on:** September 29, 2005, 03:50:04 PM »

Hmm.. OK. I looked at the World and noticed the swamps had hills in them, and the US is definitely better, but still I can see that things don't exactly line up.

It could be error in the topo map, or it could be the 3d data. How does GPS data you collect in the area line up with either?

I noticed that Terraserver for some reason isn't serving up a few topo tiles in that area. That doesn't make me trust the topo alignment completely.

I don't think the elevation data is down to 0.1 meters. The resolution is, at best, 30 meter -- but that's the grid spacing. But in general the difference between possible elevation data is much higher resolution for the elevation data than for the topo lines -- which are often at intervals of 20 ft, 40 ft, etc.

The answer might just be that they are from different sources and there isn't a definitive answer on what the topography looks like in that area. Maybe it changes over time? It isn't the case that the elevation data was made FROM the topo maps, so I wouldn't expect an exact correspondence. In mountianous regions they seem to agree fairly closely.

You might try downloading a 30 meter or 10 meter resolution DEM file for that area to compare to the auto-downloaded one.

Good luck.