[OSGeo Africa] How to load .ort DEM into QGIS

Melissa Akoue Mve melakouemve at gmail.com
Tue Apr 28 05:30:52 PDT 2015


Hi All,

Thank you for this feedback.

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).

Thanks again for all your help.

Kind Regards,
Melissa

2015-04-28 14:05 GMT+02:00 Wolfgang Luck <wolfluck at mweb.co.za>:

> Hi
> 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.
> The .ort files are not always formatted as a clean regular lattice.
>
> 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.
>
> 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.
>
> The cheapest and best DEM available commercially is SUDEM from the
> University of Stellenbosch with a 5m posting.
>
> 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.
>
> With kind regards
>
> Wolfgang
>
> On 2015/04/28 11:25 AM, africa-request at lists.osgeo.org wrote:
>
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>> Today's Topics:
>>
>>     1. Re: How to load .ort DEM into QGIS (Gavin Fleming)
>>
>>
>> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Tue, 28 Apr 2015 10:55:36 +0200
>> From: Gavin Fleming <gavinjfleming at gmail.com>
>> To: africa at lists.osgeo.org
>> Subject: Re: [OSGeo Africa] How to load .ort DEM into QGIS
>> Message-ID: <553F4B08.2060203 at gmail.com>
>> Content-Type: text/plain; charset="utf-8"
>>
>> Thanks Melissa
>>
>> The NGI DEM is probably a lot more accurate (and higher resolution) than
>> SRTM so it would be worth scripting that GRASS process if you're
>> concerned about accuracy.
>>
>> The NGI DEM if I remember correctly is derived from stereo orthophotos,
>> more recent ones being done 100% digitally. SRTM on the other hand has
>> many artefacts. I've used this fixed-up SRTM data before:
>> http://www.viewfinderpanoramas.org/dem3.html.
>>
>> Gavin
>>
>> On 28/04/2015 08:11, Melissa Akoue Mve wrote:
>>
>>> Hi Gavin and Walter,
>>>
>>> Sorry for my late response.  I got caught in meetings and only managed
>>> to try out your suggestions late on friday and this morning.
>>>
>>> I tried both ways and they both work well. Walter's suggestion of
>>> downloading the SRTM 30m DEMs is the faster method because I can
>>> download the images as geotiffs and load them in directly without
>>> Grass or any other manipulation.  However I have learnt a lot from
>>> trying Gavin's method too.
>>>
>>> Thank you both for your invaluable assistance.  This discussion group
>>> is very valuable tool.
>>>
>>> Regards,
>>> Melissa
>>>
>>> 2015-04-24 13:50 GMT+02:00 Jean Paul Ngalams <jpngalams at gmail.com
>>> <mailto:jpngalams at gmail.com>>:
>>>
>>>      Thank you Gavin for your answer to Melissa's question; I had the
>>>      same preoccupation.
>>>
>>>      On Fri, Apr 24, 2015 at 12:17 PM, Gavin Fleming <gavin at kartoza.com
>>>      <mailto:gavin at kartoza.com>> wrote:
>>>
>>>          Hi Melissa
>>>
>>>          I've just done one of the NGI ort files (2631AA07.ort) in
>>>          GRASS (result below). Pretty much everything you need is at
>>>          http://grasswiki.osgeo.org/wiki/Import_XYZ. Note there are
>>>          multiple spaces so you'll have to use the trick on that page
>>>          to collapse multiple spaces to single spaces. Also note the
>>>          coordinates are south facing TM so you'll have to do one zone
>>>          at a time then project them together, or project all to UTM or
>>>          Albers first.
>>>
>>>
>>>
>>>          You should be able to import directly in QGIS (which uses
>>>          GDAL) after re-ordering the data as per
>>>          http://www.gdal.org/frmt_xyz.html. However on this one I
>>>          tried, GDAL thinks the number of lines doesn't match the size
>>>          of the grid it has calculated so it fails., so I just switched
>>>          to GRASS.
>>>
>>>          Good luck and let us know when you've done it (and how you did
>>>          it).
>>>
>>>          Gavin
>>>
>>>
>>>          On 24/04/2015 11:09, Melissa Akoue Mve wrote:
>>>
>>>>          Hi All,
>>>>                    I need to do a slope analysis of the whole Western
>>>> Cape
>>>>          area.   I had planned to do this with the raster tool set in
>>>>          QGIS but after getting the DEM's from the NGI I discovered
>>>>          their DEMs are in a .ort format which is not a standard GDal
>>>>          format.
>>>>                    I was informed that the only way to load this .ort
>>>> into QGIS
>>>>          is by converting it to a CSV and then loading the CSV file
>>>>          into QGIS, and then converting it to raster when in DEM.   I
>>>>          have tried this but it is laborious and will take too long
>>>>          because there are more than 50 NGI DEMs that make up the
>>>>          Western Cape area.
>>>>                    Does anyone know of a simpler way to load the .ort
>>>> file.   I
>>>>          have searched for ways to convert the .ort to a geotiff but I
>>>>          havent been successful yet,
>>>>                    Kind Regards,
>>>>          Melissa Akoue Mve,
>>>>
>>>>
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>>>
>>>          --
>>>
>>>  ------------------------------------------------------------------------------------------
>>>          Gavin Fleming Joint MD PrGISc [PGP1234]
>>>          Visit http://kartoza.com <http://kartoza.com/>to find out
>>>          about open source:
>>>          * Desktop GIS programming services
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>>>          Skype: phlemingo
>>>
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>>>
>>>      --
>>>      Jean Paul NGALAMULUME TSH.
>>>      Contact:+243 9 97 53 47 59
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> --
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