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<div class="moz-cite-prefix">On 2/16/21 10:30 AM, Maris Nartiss
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAK8GbeC2nYb6R+nBA=49jE9Kfd4mWPVZ1SViHaErXLwocw5QJg@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">Hello Dave,
your case is even easier – TIFF and ASC both are raster formats and
thus it is a simple task: import -> contour -> export (skipping all
point cloud related tweaking).</pre>
</blockquote>
<p><br>
</p>
<p>I'd like to just back what Maris already suggested: Try working
directly in GRASS. It does have a somewhat steep learning curve at
the beginning, but if you stick with it, you'll find in the long
run that it pays ("with interest"). You'll be able to perform
complex analyses on multiple maps relatively easily.</p>
<p><br>
</p>
<p>For example, if you post the name of the Lidar based elevation
raster that you have, someone on the list will probably help to
format the commands that Maris listed above into a detailed recipe
to get elevation contours.<br>
</p>
<p><br>
</p>
<blockquote type="cite"
cite="mid:CAK8GbeC2nYb6R+nBA=49jE9Kfd4mWPVZ1SViHaErXLwocw5QJg@mail.gmail.com">
<pre class="moz-quote-pre" wrap="">
Create a new location. Either select coordinate system directly or use
one from TIFF file. Import data with r.in.gdal – don't forget to
specify "-e" parameter – it will align the computational region with
the imported raster. Then proceed to use r.contour followed by
v.out.ogr.
In future – do not ask for help with LiDAR but ask for help with
raster data. Otherwise you'll get advices like you got from me – how
to work with point clouds although you are looking for how to work
with ordinary rasters.
Māris.
2021-02-15 22:58 GMT+02:00, Dave Marshall <a class="moz-txt-link-rfc2396E" href="mailto:43carnaby@gmail.com"><43carnaby@gmail.com></a>:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Maris,
Many thanks for your detailed reply. My LIDAR files are not in LAS format -
they are a mixture of ASC and TIF.
I spent a long time learning how to use QGIS and don't want to have to
repeat the process with GRASS unless I have to. If there isn't a simple way
to get r.contour to work from within the later versions of QGIS, then I'll
keep on using the old version as the solution requiring the least effort.
>From your comments, it would seem that it is how QGIS imports the LIDAR
data which has changed and this is why I see the problem I reported. I also
realise that QGIS is a global application whereas my work is restricted to
the UK using the Ordnance Survey grid so I can't expect a huge resource to
look at a narrow application.
Cheers,
Dave
On Mon, 15 Feb 2021 at 10:08, Maris Nartiss <a class="moz-txt-link-rfc2396E" href="mailto:maris.gis@gmail.com"><maris.gis@gmail.com></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Hello Dave,
QGIS hides a bit of GRASS complexity by making a guess for various
parameters. As with any guess – sometimes it works, sometimes it is a
miss (and user has no idea which is the case).
To get contours out of LAS files:
1) create a location with coordinate system matching one used by LAS
files (be ware – you might need to know it in advance from metadata as
LAS files quite often lack this information);
2) create a mapset for the area of interest (could be whole region or
a single file in case of parallel processing);
3) start GRASS in newly created mapset;
4) set up your computational region (this is most important part!)
with g.region. Don't forget to choose appropriate resolution.
a) if you know the extent in advance (e.g. from a map sheet grid) use
that;
b) if you don't know the extent in advance, use actual extent from the
LAS file. I would advocate to use r.in.lidar -s and set the extent
manually with g.region – you can “snap“ your raster to coordinates.
5) import data with r.in.lidar;
6) run r.contour on the map;
7) export with v.out.ogr to Shapefile (#teamshapefile).
Good luck,
Māris.
P.S. When you wander into area of 66000 LAS files occupying nice 14T
on your disk, only a few adjustments need to be done + a bit of Python
coding + a bit of cluster management :D
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">_______________________________________________
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<a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/grass-user">https://lists.osgeo.org/mailman/listinfo/grass-user</a>
</pre>
</blockquote>
<pre class="moz-signature" cols="72">--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918</pre>
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