[Qgis-user] Extracting Raster values (time series) using polygons

[Michael Treglia]

Hi Sam,

Important point of clarification: Do you want all of the individual pixel
values? Or aggregate statistics, like min/max/mean/med...?

For the latter, there are a few tools you can use. I typically use a tool
in the Processing Toolbox, implemented in SAGA: Go to Processing Toolbox -
> SAGA -> Shape-Grid -> Grid Statistics and you'll be able to get those
stats for your desired grids. The only downside, if i recall correctly, is
that the column names don't necessarily indicate which layer the stats are
from, if you're operating on multiple rasters at a time.  There's a  'Zonal
Statistics'  tool also, which I think works well too. It is a plugin - it
is a 'core plugin', which means it come with QGIS,though you might need to
activate it (can't remember). If it is already activated, you'll see it
when you go to the 'Raster' toolbar menu.  If you don't see it there, you
can search for it in the 'Plugin' dialogue and make sure the box next to it
is checked.

(more details on plugins here:
https://docs.qgis.org/2.8/en/docs/user_manual/plugins/plugins.html)

If you want the former, that might be a bit more complicated, and is
something I typically do directly in R, but could give advice here too.

hope that helps!
mike

On Mon, Oct 5, 2015 at 1:39 PM, Sam Almond <sam_almond at hotmail.com> wrote:

> Dear QGIS Users,
>
>
>
> I am new to using qGIS and can’t find an obvious solution to my problem.
>
>
>
> I have created a time series of NDVI rasters covering my area of interest.
> The NDVI time series is derived from the Landsat imagery spanning a decade
> of so. I also have a created a vector layer, consisting of polygons, which
> represents digitized farmed fields (70 or so). The plan is to use the
> polygons to extract the raster values from the time series, ideally one
> year at a time. The ideal scenario is to be provided with the pixel values
> per field (polygon) per NDVI over the year. Is this possible? Help
> appreciated.
>
>
> Many thanks
>
>
> Sam
>
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