Re: [Qgis-user] Zonal Statistics multiple classe

[Nikos Alexandris]

Basically I am looking to getting the total cover and percent cover 
for each class (the classes are crop data), for each polygons 
(drainage area).  So basically, how many square km of corn, forest, 
wheat... do I have in each drainage area polygons? Does that make 
more sense for you?


The modules mentioned are a perfect fit for this task.


Nicolas, if you are still on it, and didn't figure it out yet,
and you could share some sample data, I could prepare something tonight.

Nikos


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Re: [Qgis-user] Zonal Statistics multiple classe

[Kirk Schmidt]

Hi Nicolas:

I would use the SAGA Raster Statistics for polygons tool which which you 
can access through the processing toolbox.




On 2/12/2018 5:38 AM, Nikos Alexandris wrote:
* Nicolas Cadieux  [2018-02-11 23:37:13 
-0500]:



Hi,
I have a raster (int16) with multiple class (ex 1 = grass, 2 = 
trees...) and a vector polygon file.  I want to have the zonal 
statistic (cum, min, max, sum...) for every class.  What is the best 
way to get this done in QGis 2.18.16 on Windows 10?


Thanks,
Nicolas


Nicolas,

not a direct answer, yet if you have access to GRASS' tools, then you 
could look

into v.rast.stats.html [0] (or even r.stats.zonal [1] if you have a
raster version of your "polygons").

If I understand correctly, you want to get statistics per polygon, not
per class. I.e., min, max, sum and the rest, for each class inside
each vector polygon.

v.rast.stats will derive the statistics in question from a raster map
for the parts covered by the vector map. Note, internally, the vector 
map is

converted to a raster.

If you have two rasters, then your base map is the "multiple classes 
map".
Your cover map will be the "rasterised vector polygons", which you 
consider as

the zones to compute the statistics in question within. Very useful,
here, is the `-r` flag: "Create reclass map with statistics as category
labels".

Nikos

[0] https://grass.osgeo.org/grass74/manuals/v.rast.stats.html
[1] https://grass.osgeo.org/grass74/manuals/r.stats.zonal.html


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Re: [Qgis-user] Zonal Statistics multiple classe

[Nikos Alexandris]

(Cc in the list, someone might pick this up and see why you can't go
through)

Some examples in the end using r.stats.zonal. I think they are useful
for your working case. I am sorry that I don't include specific examples for
v.rast.stats. But they don't differ much.

You could convert your vector polygons in a raster map. Think about the
resolution of the computational region before doing so.


* Nicolas Cadieux  [2018-02-12 10:04:33 -0500]:


Hi Nikos,
I am having problems replying to the user list.  The Email comes back 
to me as unsent (I need to contact the list admin). I am therefore 
replying using your email address.  Can you confirm that you got this 
email.

Sorry if you received multiple copies.
Nicolas


Thanks for your help,

I started playing with the v.rast.stats but with difficulty.  I Will 
work on it more today.  I figured it may have to do with the -r flag.


How can I convert the int16 to a category format in grass if I was not 
using the -r flag?  I am trying to understand the data type in Grass 
for the category format?  I was hoping int16 would cover it.


I am sure you know all of the following, but just to be on the same ground:

GRASS understands all integers (automatically). If your raster map is of
type integer, you don't need to do anything about it. Just take care to
set the computational region using `g.region`.  This is something I
(still) frequently forget. The region is important for when using
`r.report`! See also: https://grasswiki.osgeo.org/wiki/GRASS_raster_semantics.

With the `r.category` module one can associate category values (in which
case "integer raster values") and category labels. See the manual for
details and examples.



“If I understand correctly, you want to get statistics per polygon, not

per class. I.e., min, max, sum and the rest, for each class inside
each vector polygon. “


Basically I am looking to getting the total cover and percent cover 
for each class (the classes are crop data), for each polygons 
(drainage area).  So basically, how many square km of corn, forest, 
wheat... do I have in each drainage area polygons? Does that make more 
sense for you?


The modules mentioned are a perfect fit for this task.

All you need is to import in GRASS your raster map with (integer) land
cover categories and your vector map.  The land cover map will be the
"cover". The vector map, in your case, will be the base.

I was hoping the LecoS plugin would work but I think It need to work 
with one crop type at a time.  Working on that too!  Patience is a 
virtue... I guess!


Thanks for your help
Nicolas


Thanks to the GRASS community.

Compute and report (and even output as an ASCII file) zonal statistics per land
cover category:

```
r.stats.zonal method=average base=landcover cover=temperature 
output=average_temperature_per_landcover -r
r.report average_temperature_per_landcover -nh units=me,c,p --q 
output=zonal_statistics_report
```

Sample report where the 1, 2, 10, .., 50 are land cover classes:

+-+
| Category Information   |   square|   cell|   %  |
| #|description  |   meters|  count| cover|
|-|
| 1|28.447180. . . . . . . . . . . . . . . . . . |   25,967,465|  28853|  0.80|
| 2|38.517164. . . . . . . . . . . . . . . . . . |3,245,371|   3606|  0.10|
|10|30.085036. . . . . . . . . . . . . . . . . . |  289,801,877| 322005|  8.94|
|15|31.038448. . . . . . . . . . . . . . . . . . |   16,441,051|  18268|  0.51|
|20|26.774910. . . . . . . . . . . . . . . . . . |2,630,504,294|2922809| 81.11|
|25|29.068074. . . . . . . . . . . . . . . . . . |   28,675,540|  31862|  0.88|
|30|38.678536. . . . . . . . . . . . . . . . . . |   38,565,551|  42851|  1.19|
|35|37.814765. . . . . . . . . . . . . . . . . . |   17,395,043|  19328|  0.54|
|40|33.512648. . . . . . . . . . . . . . . . . . |  132,926,997| 147698|  4.10|
|41|-nan . . . . . . . . . . . . . . . . . . . . |  112,499|125|  0.00|
|45|35.253821. . . . . . . . . . . . . . . . . . |4,467,560|   4964|  0.14|
|50|39.191611. . . . . . . . . . . . . . . . . . |   54,872,503|  60970|  1.69|
|-|
|TOTAL   |3,242,975,751|3603339|100.00|
+-+


Compute without the -r flag and report:

```
r.stats.zonal method=average base=landcover cover=temperature 
output=average_temperature_per_landcover
r.report average_temperature_per_landcover -nh units=me,c,p --q 
output=zonal_statistics_report
```

Sample output where the categories are temperature strata:

+-+
|   Category Information

Re: [Qgis-user] Zonal Statistics multiple classe

[Nikos Alexandris]

* Nikos Alexandris  [2018-02-12 10:38:42 +0100]:


* Nicolas Cadieux  [2018-02-11 23:37:13 -0500]:


Hi,
I have a raster (int16) with multiple class (ex 1 = grass, 2 = 
trees...) and a vector polygon file.  I want to have the zonal 
statistic (cum, min, max, sum...) for every class.  What is the best 
way to get this done in QGis 2.18.16 on Windows 10?


Thanks,
Nicolas


Nicolas,

not a direct answer, yet if you have access to GRASS' tools, then you could look
into v.rast.stats.html [0] (or even r.stats.zonal [1] if you have a
raster version of your "polygons").

If I understand correctly, you want to get statistics per polygon, not
per class. I.e., min, max, sum and the rest, for each class inside
each vector polygon.

v.rast.stats will derive the statistics in question from a raster map
for the parts covered by the vector map. Note, internally, the vector map is
converted to a raster.

If you have two rasters, then your base map is the "multiple classes map".
Your cover map will be the "rasterised vector polygons", which you consider as
the zones to compute the statistics in question within. Very useful,
here, is the `-r` flag: "Create reclass map with statistics as category
labels".



Yet, I have to correct myself here. It's the other way around. 
The cover map is the source of numbers to compute the statistics.

Thus, here it will be the "multiple classes map".

The base map will be the "polygons" map.

Nikos



[0] https://grass.osgeo.org/grass74/manuals/v.rast.stats.html
[1] https://grass.osgeo.org/grass74/manuals/r.stats.zonal.html





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Re: [Qgis-user] Zonal Statistics multiple classe

[Nikos Alexandris]

* Nicolas Cadieux  [2018-02-11 23:37:13 -0500]:


Hi,
I have a raster (int16) with multiple class (ex 1 = grass, 2 = 
trees...) and a vector polygon file.  I want to have the zonal 
statistic (cum, min, max, sum...) for every class.  What is the best 
way to get this done in QGis 2.18.16 on Windows 10?


Thanks,
Nicolas


Nicolas,

not a direct answer, yet if you have access to GRASS' tools, then you could look
into v.rast.stats.html [0] (or even r.stats.zonal [1] if you have a
raster version of your "polygons").

If I understand correctly, you want to get statistics per polygon, not
per class. I.e., min, max, sum and the rest, for each class inside
each vector polygon.

v.rast.stats will derive the statistics in question from a raster map
for the parts covered by the vector map. Note, internally, the vector map is
converted to a raster.

If you have two rasters, then your base map is the "multiple classes map".
Your cover map will be the "rasterised vector polygons", which you consider as
the zones to compute the statistics in question within. Very useful,
here, is the `-r` flag: "Create reclass map with statistics as category
labels".

Nikos

[0] https://grass.osgeo.org/grass74/manuals/v.rast.stats.html
[1] https://grass.osgeo.org/grass74/manuals/r.stats.zonal.html


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