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| Error between DEM and Topo Map https://forum.transdem.de/viewtopic.php?f=7&t=460 |
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| Author: | activerail [ 12 Sep 2016 19:45 ] |
| Post subject: | Error between DEM and Topo Map |
This has occurred before but not as much as current project. I'm working on the now defunct Lowe Mt. Railway in Southern California. Downloaded NED data from USGS with no problem. Downloaded and geopositioned two topo maps. Traced the route and created a route file. When exporting all to Trainz 4.2 I find that the route is about meters off. The reason I can recognize this is that the streams/stream beds are 50 metes off. So, my question, is there any way I can make an adustment to the topo maps to better align them with the DEM. Hope this is clear. Thanks. |
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| Author: | Jerker [ 13 Sep 2016 00:58 ] |
| Post subject: | Re: Error between DEM and Topo Map |
G'day activerail, ...that's rather odd! Are you sure you properly georeferenced the Topo maps? You don't state the source fro these but if they aren't from the USGS, then you must make sure that they are georeferenced in the correct projection. Having said that, it is not possible to shift the Topo map to make it more closely match the DEM in this circumstance (this functionality, however, is an addition I have been pushing for since day one). On the other hand, it IS possible to shift the DEM to match the Topo maps. The method to achieve this is discussed in the excellent Help file and tutorials that accompany the program (in the "Menu Item: Shift DEM" section on page 84) ... ...to accomplish this task, I set up two control points prior to opening the "Shift DEM" dialogue box. Place one point on a known land mark point on the Topo map (such as the peak of a mountain) and place the second on the same point in the DEM. Open the dialogue and and click the [Last Control points distance] button and then [OK]. If the DEM shifts the wrong way, undo the action and the next time, click the [Invert] button and you should get it right. It is worth noting that the the closer you zoom into the work whilst selecting the Control Points, the more accurate the shift will be... Jerker {:)} |
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| Author: | activerail [ 16 Sep 2016 17:50 ] |
| Post subject: | Re: Error between DEM and Topo Map |
Thanks, Jerker, for you input. Will look at shifting the DEM. I have been using geopositioned .pdf for the USGS website which only require changing them to UTM. The 2 maps I've used were dated 1928 and 1939. I've subsequently used Google Earth images to follow the route laid out with the previous topo overlays. This has created a route that is right on. The problem is that most of the routes that I've been working on are no defunct and their routes are only "scars" on the terrain when I attempt to use Google Earth. Gary Orr Coarsegold, CA, USA |
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