TransDEM Forum

Hi, me again with another question. I know how to import a map image into Transdem, but what I want to know is if I dont have any known reference points on the image but I do know where it fits in on the dem, is it possible to manually place it where it should be? Also the image is at a different scale so is Transdem capable of correcting its scale once its position is known? The image I have in mind is at a railway junction so can be located fairly precisely but unless I overlay it onto something like Google Earth first, I cant see how to do it manually.

G'day AntonyVW,

Indeed, Antony, it is possible to do that which you seek. The easiest way I would recommend doing it would be to determine the geographic co-ordinates of the South West and North East corners of the Raster map and to then manually enter these values into the appropriate places using the [SHIFT]+[R] (Menu>Raster Map>Georeference With Map Corners Only..) georeferencing option available once the image is 'loaded'. TransDEM will then 'automatically' adjust the image so as to make it fit the correct position, according to the latitudes and longitudes entered. Keep in mind that the Raster Map SHOULD already be in a correct UTM projection and have North pointing upwards, otherwise this method WILL introduce major errors (which manifest themselves mainly as errors in the 'placement' of Topographic data on the map relative to the DEM within the area of the map)...

...with regard to the 'resolution' of your DEM data, if, as I presume correctly, you are using the 3 (90 meter) arcsec SRTM data, you should find that 'better' results will be forthcoming by using the 1 arcsec (30 meter) Aster GDEM data...

Jerker [:)}

Thanks Jerker once again. What I have is a series of small maps which I have overlaid together to give me a track for my local station layout based around 1905. It clearly shows me where the cuttings and embankments are and would assist me tremendously. My only problem left then is to work out the exact track elevation at given points. I note on the map segments that there are, at intervals, 2 numbers, one above the other. For example 69 above 1.195. They often have an arrow pointing to the center of the two tracks, and or a curly line across the tracks (like an elongated letter 'S'). Im presuming that one of these numbers is the elevation - possibly the top one in feet - and the lower one is the inclination of the line? Remembering of course that this map is an old map so measurements would be in feet and inches. My only problem with this is that I can also see fairly close to the track a BM marks, which I know are spot heights, which give a very different height reading. So a little confused over this. Still progress is being made and although I spent most of yesterday reading all sorts of documents related to Transdem and dems etc instead of building maps tiles, I think I it was productive. Thanks again

Historic maps are always an interesting challenge. As Jerker has pointed out, there are basically two options for manual georeferencing in TransDEM: Map corners (2 points), or 3(+1) points on the map itself.

In this case, 3(+1) is probably the way to go. (I guess you used 2 point corner georeferencing for your Open Data O/S maps.) Precondition is always that the map is to scale. Assuming that is the case, you should try to identify 3 points near the edges of the historic map which you can find back on a modern map or an an aerial image. The modern map or the aerial image will then provide the coordinates of those points.


@Jerker: There is 50 m terrestrial DEM data available for Britain for free, called O/S Land-form Panorama.

So far I have not attempted to do that (grandchildren visiting). I have tried overlaying it onto google earth and importing that. Lol - my measurements were not as accurate as I had hoped as the junction was slightly north west of where I wanted it but not by much.

If you tried to add your add to Google Earth, do it the other way round. Add it to TransDEM, only using GE coordinates for those points you could identify. TransDEM may have a bit more flexibility here.