[GRASS-user] r.contour

[Dave Marshall]

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 <maris.gis at gmail.com> wrote:

> 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
>
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