[Qgis-user] Select heights from a raster layer

[Neumann, Andreas]

Hi Claus, 

Glad it helped. 

About the bands: raster files can have several bands (or channels). This
is quite typical in remote sensing. Satellite images often have several
bands recorded in different spectrums of the light (e.g. visible
spectrums, infrared, ultraviolett, etc.). You could also store several
height informations at the same geographic point into the same raster
file. One band could hold the surface model, the other the terrain model
and a third band could hold the ground water level model. A typical
orthoimage has three bands in the visible spectrum of the light: one for
red, green and blue channel. 

Think of a band as a of a layer in a raster file. 

Andreas 

On 2016-03-11 07:52, Claus Backalarz wrote:

> Hi Andreas (and others who have answered our questions), "Zonal Statistics" do not exactly do the job, it also do the next step in our task! 
> 
> In fact our intentions was to select the surface heights (measured from a flight with laser in a 1,6mx1,6m grid) only inside the buildings polygons and then export the roof heights (still in a 1,6mx1,6m grid ) to a another program. In this program the mean heights of all buildings should be calculated and attached to the buildings. 
> 
> Now we can use "Zonal Statistics" (and only select the mean-calculation) and then we have the mean height as a new feature value for each building. Perfect! 
> 
> We did tried the "Zonal Statistics" earlier but couldn't comprehend were the calculated values ends (there was a cryptic "Band"-drop down in "Zonal Statistics" form). 
> 
> I still don't understand the "Band"-drop down, but so what, it works. 
> 
> Thank you very much. 
> 
> Venlig hilsen / Best regards
> Claus Backalarz 
> Senior specialist
> DELTA Acoustics 
> 
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> FROM: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] ON BEHALF OF Neumann, Andreas
> SENT: 10. marts 2016 17:46
> TO: Richard McDonnell
> CC: qgis-user at lists.osgeo.org
> SUBJECT: Re: [Qgis-user] Select heights from a raster layer 
> 
> Hi, 
> 
> Have you tried the "Zonal Statistics plugin"? 
> 
> After enabling this plugin, you can find it in the raster menu - or maybe it is already there by default. 
> 
> It calculates count, sum, min, max, mean at the intersection of a polygon layer (e.g. your polygon layer with building footprints) and a given raster layer (your grid based terrain model). 
> 
> I think it does what you want. 
> 
> Hope this helps, 
> 
> Andreas 
> 
> On 2016-03-10 17:37, Richard McDonnell wrote: 
> 
> Claus,
> It may not be exactly what you want, but one suggestion would be to derive the centroids from the building polygons (making sure there within the polygon) and then use the point sampling tool to pull the elevations from your DSM. of course that will not guarantee that you get the max height
> If you can, it may be best to go back to the DSM source, the raw data used to derive the DSM. If it is, for instance a point cloud dataset, like that from Lidar, you could select all points within your polygons, then extract only the points at the apex (first returns), just a thought. 
> Regards,
> 
> Richard
> 
> On 10/03/2016 11:52, Claus Backalarz wrote: 
> 
> Hi, we are novices in the QIS-world so bear with misunderstandings and misconceptions. 
> 
> We have a raster layer with surface heights called DSM_611_86. Originally it is was a plain text file with a header describing number of rows and columns, geographically lower left corner etc. Data was the surface heights in 6250 rows and 6250 columns. The file was imported without problems with "Add raster layer" in QGIS. It is beautiful showed in QGIS2.14 Essen on Windows 7. So far no problem. 
> 
> But we want with (a lot) of polygons representing buildings to select all surface heights inside this polygons. The selected heights should afterward be saved as a reduced raster layer or, even better, as a shape file of points with an attribute for the heights. In other words as a layer of points with one and only one attribute, the surface height. 
> 
> We have tried the plug-in "Point Sampling Tools" and some other hints from QGIS home pages, but I seems that select and save heights in the described way is "out of scope". 
> 
> Do anybody have a useful way out of this problem ? 
> 
> Best regards 
> 
> Claus B 
> 
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