<div dir="ltr"><div><div><div>Hi All,<br><br>Thank you for this feedback.<br><br></div>For the moment, the DEM does not need to be too accurate as I simply need to locate mountaneous areas in the Western Cape with a slope of more than 12 degrees. I feel the SRTM 30m DEMs should be fine for this initial analysis (although I could be wrong, but I am sure the results will soon show me if I am). <br><br></div><div>Thanks again for all your help.<br><br></div>Kind Regards,<br></div>Melissa<br></div><div class="gmail_extra"><br><div class="gmail_quote">2015-04-28 14:05 GMT+02:00 Wolfgang Luck <span dir="ltr"><<a href="mailto:wolfluck@mweb.co.za" target="_blank">wolfluck@mweb.co.za</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi<br>
Please take note that the .ort files are very heterogenious in nature. Andre Brytenbach has written a conversion tool to ASCII raster delivered to CDNGI, while he was still at SAC years ago.<br>
The .ort files are not always formatted as a clean regular lattice.<br>
<br>
In general they have a 20m posting and there is an issue with pixel as area or pixel as point referencing. Find out from CDNGI which convention they are adhering to. I can remember that the data was offset by half a pixel.<br>
<br>
The 20m points have in some instances been extracted from stereo aerial photography, and that only in steep terrain. In other areas they have been interpolated from 20m contours or in flat terrain interpolated in general.<br>
<br>
The cheapest and best DEM available commercially is SUDEM from the University of Stellenbosch with a 5m posting.<br>
<br>
The WorldDEM available from Airbus is probably the most consistent product and accurate to within 1m in many areas as noted from interrogation with LIDAR data, although the specs are much more conservative.<br>
<br>
With kind regards<br>
<br>
Wolfgang<br>
<br>
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Today's Topics:<br>
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1. Re: How to load .ort DEM into QGIS (Gavin Fleming)<br>
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----------------------------------------------------------------------<br>
<br>
Message: 1<br>
Date: Tue, 28 Apr 2015 10:55:36 +0200<br>
From: Gavin Fleming <<a href="mailto:gavinjfleming@gmail.com" target="_blank">gavinjfleming@gmail.com</a>><br>
To: <a href="mailto:africa@lists.osgeo.org" target="_blank">africa@lists.osgeo.org</a><br>
Subject: Re: [OSGeo Africa] How to load .ort DEM into QGIS<br>
Message-ID: <<a href="mailto:553F4B08.2060203@gmail.com" target="_blank">553F4B08.2060203@gmail.com</a>><br>
Content-Type: text/plain; charset="utf-8"<span class=""><br>
<br>
Thanks Melissa<br>
<br>
The NGI DEM is probably a lot more accurate (and higher resolution) than<br>
SRTM so it would be worth scripting that GRASS process if you're<br>
concerned about accuracy.<br>
<br>
The NGI DEM if I remember correctly is derived from stereo orthophotos,<br>
more recent ones being done 100% digitally. SRTM on the other hand has<br>
many artefacts. I've used this fixed-up SRTM data before:<br>
<a href="http://www.viewfinderpanoramas.org/dem3.html" target="_blank">http://www.viewfinderpanoramas.org/dem3.html</a>.<br>
<br>
Gavin<br>
<br>
On 28/04/2015 08:11, Melissa Akoue Mve wrote:<br>
</span><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">
Hi Gavin and Walter,<br>
<br>
Sorry for my late response. I got caught in meetings and only managed<br>
to try out your suggestions late on friday and this morning.<br>
<br>
I tried both ways and they both work well. Walter's suggestion of<br>
downloading the SRTM 30m DEMs is the faster method because I can<br>
download the images as geotiffs and load them in directly without<br>
Grass or any other manipulation. However I have learnt a lot from<br>
trying Gavin's method too.<br>
<br>
Thank you both for your invaluable assistance. This discussion group<br>
is very valuable tool.<br>
<br>
Regards,<br>
Melissa<br>
<br>
2015-04-24 13:50 GMT+02:00 Jean Paul Ngalams <<a href="mailto:jpngalams@gmail.com" target="_blank">jpngalams@gmail.com</a><br></span>
<mailto:<a href="mailto:jpngalams@gmail.com" target="_blank">jpngalams@gmail.com</a>>>:<span class=""><br>
<br>
Thank you Gavin for your answer to Melissa's question; I had the<br>
same preoccupation.<br>
<br>
On Fri, Apr 24, 2015 at 12:17 PM, Gavin Fleming <<a href="mailto:gavin@kartoza.com" target="_blank">gavin@kartoza.com</a><br></span><span class="">
<mailto:<a href="mailto:gavin@kartoza.com" target="_blank">gavin@kartoza.com</a>>> wrote:<br>
<br>
Hi Melissa<br>
<br>
I've just done one of the NGI ort files (2631AA07.ort) in<br>
GRASS (result below). Pretty much everything you need is at<br>
<a href="http://grasswiki.osgeo.org/wiki/Import_XYZ" target="_blank">http://grasswiki.osgeo.org/wiki/Import_XYZ</a>. Note there are<br>
multiple spaces so you'll have to use the trick on that page<br>
to collapse multiple spaces to single spaces. Also note the<br>
coordinates are south facing TM so you'll have to do one zone<br>
at a time then project them together, or project all to UTM or<br>
Albers first.<br>
<br>
<br>
<br></span><div><div class="h5">
You should be able to import directly in QGIS (which uses<br>
GDAL) after re-ordering the data as per<br>
<a href="http://www.gdal.org/frmt_xyz.html" target="_blank">http://www.gdal.org/frmt_xyz.html</a>. However on this one I<br>
tried, GDAL thinks the number of lines doesn't match the size<br>
of the grid it has calculated so it fails., so I just switched<br>
to GRASS.<br>
<br>
Good luck and let us know when you've done it (and how you did<br>
it).<br>
<br>
Gavin<br>
<br>
<br>
On 24/04/2015 11:09, Melissa Akoue Mve wrote:<br>
</div></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div class="h5">
Hi All,<br>
I need to do a slope analysis of the whole Western Cape<br>
area. I had planned to do this with the raster tool set in<br>
QGIS but after getting the DEM's from the NGI I discovered<br>
their DEMs are in a .ort format which is not a standard GDal<br>
format.<br>
I was informed that the only way to load this .ort into QGIS<br>
is by converting it to a CSV and then loading the CSV file<br>
into QGIS, and then converting it to raster when in DEM. I<br>
have tried this but it is laborious and will take too long<br>
because there are more than 50 NGI DEMs that make up the<br>
Western Cape area.<br>
Does anyone know of a simpler way to load the .ort file. I<br>
have searched for ways to convert the .ort to a geotiff but I<br>
havent been successful yet,<br>
Kind Regards,<br>
Melissa Akoue Mve,<br>
<br>
<br>
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