[OSGeo Africa] How to load .ort DEM into QGIS
Wolfgang Luck
wolfluck at mweb.co.za
Tue Apr 28 05:05:48 PDT 2015
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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> 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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>> -------------------------------------------------------------------------------------------
>> Kartoza = Linfiniti ? Afrispatial
>>
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>>
>> --
>> Jean Paul NGALAMULUME TSH.
>> Contact:+243 9 97 53 47 59
>> ngalamjp at yahoo.fr <mailto:ngalamjp at yahoo.fr>
>> Skype:jp.ngalams
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--
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