[GRASS-user] r.contour

[Dave Marshall]

Micha & Maris,

Thanks to both of you for your help and suggestions. I had a go with GRASS
this evening and managed to load a LIDAR tif file
(NT02SW_50CM_DTM_PHASE3.tif) and run r.contour although I couldn't get the
output to appear on the raster image. Some of the key tasks I need to be
able to do are to add grid lines, colour the images based on height in a
flexible way, subtract one raster image from another and to be able to
overlay the contours generated on a satellite image such as Google Earth. I
suspect that all this can be done using GRASS directly but assuming it can
be, I'm no further forward than I am at the moment using r.contour via QGIS
so I'm not sure that taking the time to learn GRASS will be worthwhile
having spent several months learning QGIS. The only really useful feature
for my work which QGIS doesn't provide is the ability to display the height
values on the image on a square by square basis, as can be done if the file
is loaded into Excel.

Cheers,

Dave

On Tue, 16 Feb 2021 at 09:58, Micha Silver <tsvibar at gmail.com> wrote:

>
> On 2/16/21 10:30 AM, Maris Nartiss wrote:
>
> 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).
>
>
> 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.
>
>
> 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.
>
>
> 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 <43carnaby at gmail.com> <43carnaby at gmail.com>:
>
> 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> <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
>
>
> _______________________________________________
> grass-user mailing listgrass-user at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/grass-user
>
> --
> Micha Silver
> Ben Gurion Univ.
> Sde Boker, Remote Sensing Lab
> cell: +972-523-665918
>
>
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