Issue converting DEM in ASCII with LO17 (apparent) projection

Question

I'm a newbie where GIS is concerned, so please don't hesitate to point me to a manual.

We've got DEM files without any projection information. One is called 000426_LO17_200M_BD16_25.ORT and contains lines like the following:

-75925.00 2710900.00   834.88
-75925.00 2710800.00   832.98
-75925.00 2710700.00   830.77
-75925.00 2710600.00   832.38
-75925.00 2710500.00   830.83

So we think this is a LO17 projection, at a 200M DEM spacing, and we're guessing the 3rd column is elevation in m. Given that we know the data is from South Africa (or close to it), this seems about right.

We're trying to get this data into Google Fusion, but need to convert it into a format that Google Fusion will understand, or equivalently, into a gdal format that we can then convert to Google Fusion.

We've tried a gdal_grid conversion, but when attempting to import into Fusion, we get Fusion is complaining about no mapping for TMSO to proj.4

If anybody has any ideas or suggestions, it would be greatly appreciated.

All the best, Craig

Answer 1

You can convert the file to Lat Lon Alt using:

invproj -f %.9f -s -m -1 +proj=tmerc +ellps=WGS84 +datum=WGS84 +k_0=1 +lon_0=17 +lat_0=0 +x_0=0 +y_0=0 +units=m <000426_LO17_200M_BD16_25.ORT >000426_LO17_200M_BD16_25.csv

ogr2ogr can read the out csv.

Answer 2

it's difficult just having a few points and not knowing anything else. To be honest, I'd be very surprised if the following is the correct solution to the problem.

What I did was to make a csv file from your five points and load it into the most recent version of QGIS (1.8). I then made a custom CRS using the following string:

+proj=tmerc +lat_0=0 +lon_0=17 +k=1 +x_0=0 +y_0=0 +axis=wsu +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

and set the layer CRS to this. I then exported the csv layer to a KML file to see where the points plotted onto the real world. But I really doubt that this is going to help you much, I'm assuming that your 'z' values are elevations and these are nowhere near right for where the points plot. Of course, had this method worked it would have provided a simple means of transforming your data, sorry. LO17 is such a peculiar projection, in fact I'm far from sure how QGIS handles the "+axis=wsu" in the parameter string.

Nick.