[Qgis-user] large DEM not loading tiles correctly

[Richard McDonnell]

Hi,
So the underlying data might not be very efficient, for example, no internal Tiling, or compressions used.
What I would suggest is…


1.       Create a VRT of all the data you have.

2.       Using GDAL Translate convert the VRT into a GeoTiff, with the following

a.       NoData Value of -9999

b.       Advanced Parameters

                                                               i.      NUM_THREADS = ALL_CPUS

                                                             ii.      BIGTIFF = YES

                                                           iii.      TILED = YES

                                                           iv.      COMPRESS = LZW

c.       Data Type to Float32

3.       You can then also use Pyramids for your Maximum zoom levels, I recommend external, as they can be deleted if you want to change them.

I hope that goes some wat to helping.
Kind Regards,

Richard.



——
Richard McDonnell MSc GIS, FME Certified Professional
FRM Data Management

——
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——
MSc GIS, FME Certified Professional

——
Oifig na nOibreacha Poiblí
Office of Public Works

Sráid Jonathan Swift, Baile Átha Troim, Co na Mí, C15 NX36
Jonathan Swift Street, Trim, Co Meath, C15 NX36
——
M +353 87 688 5964 T +353 46 942 2409
https://https://gov.ie/opw<https://www.opw.ie>

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Email Disclaimer: https://www.gov.ie/en/organisation-information/439daf-email-disclaimer/<https://www.opw.ie/en/disclaimer/>
From: Qgis-user <qgis-user-bounces at lists.osgeo.org> On Behalf Of Michael.Dodd via Qgis-user
Sent: 28 September 2022 12:32
To: 'qgis-user at lists.osgeo.org' <qgis-user at lists.osgeo.org>
Subject: [Qgis-user] large DEM not loading tiles correctly

I am trying to put together the 50m uk DEM e.g. https://www.data.gov.uk/dataset/835cf20a-8feb-4394-8b30-dcfe840ac13d/os-terrain-50-dtm but it often does not work.  The data are supplied as thousands of small tiles that takes ages to unpack and put into a folder to get into qgis, however when you try to load the tiles some work and others don’t and even the ones that do work and display fine sometimes don’t merge to form large tiles and sometimes will form virtual rasters but not merged files.  Basically it is a mess.  I suspect others have done this successfully so any suggestions?
It is particularly odd that some of the tiles will merge and others won’t and that some won’t display if you ask it to load hundreds at a time but if you just add them in a few at a time then they will load and display (in addition to the hundreds already loaded). This suggests various bugs assuming the original data are not in a whole variety of slightly different formats (should all be .asc)

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