Hi

Use zonal statistic. 

https://docs.qgis.org/2.2/en/docs/user_manual/plugins/plugins_zonal_statistics.html

Regards,

Thank you, Vahid, but how can I do this without a raster layer?

I presume you have another shapefile which represents the 'regions' within which you want to measure the cover of different classes from the shapefile you had uploaded.

If you want to rasterise the class shapefile to use zonal statistics, you can do this by going to the raster menu in QGIS and choosing the 'conversion > Rasterize' tool. Select the shapefile you want to convert to a raster, specify the column of the shapefile which contains the relevant values (code11 in your case), specify the resolution of the raster you want (25 in your case), and specify an output name for the raster. Then use the zonal statistics plugin as mentioned in Vahid's answer, along with the 'regions' shapefile.

If you want to work with vectors only, then what you will need to do is -

 Thank you so much! I"ve rasterized the class shapefile, but I don't have a shapefile with the regions I want to analyze. Do you know of any tutorial that can walk me through it? 

 I'd suggest the following steps:

1. Create SHP files for the regions you analyze 

2. Intersect SHP files with classes with this new ones.

3. For the subset merge the SHP having one and the same code

4. Calculate the area and length for each class

I guess it is one day work. 

Good luck!

Thank you, Hristo! Do you know of any tutorial that can walk me through the creation of the shp file for the regions?

Not sure. You have to do it manually digitizing over the raster from Google for example.

Alright. I have the raster, so what I have to do is delimitate the buffer zones using QGIS and create a vectorial layer from it? 

Deciding what regions you want to do your assessment within is critical.

Do you have co-ordinates around which you want to quantify the % of different classes?

If so, you could add the co-ordinates to the analysis as a shape file of points, then add a buffer around them (specified as a radius around each point) and extract data of classes which fall within them.

I can give you a clearer answer if you can provide details of what 'regions' you want to work with, e.g. are they points, or polygons, or a grid.

You can easily do statistical calculation from using the pixel size then you can find the area size of any class by multiplet it no. of pixels of that class by the pixel size.

my regards

Thank you, Varum. I do have the coordinates. The regions from which I need to get the percentages are the circular buffer zones (1000m ratio) around the places where I did the bird observations. Another author did a similar work using polygons (she says "buffer zones with each of the capture site as center where traced and an intersection permitted us to associate to each site the polygons or fractions of polygons inside those buffer zones").

Thank you, Hayden! I believe I still need to create a shp with the zones I want to analyze using this method? 

Hi Joao,

If you have your landcover rasterized and your buffer zones, then the easiest thing to do is to calculate zonal statistics within your buffers. However, I think there are two issues you have to be aware of:

- Regarding your raster, if you have converted the shapefile to raster, I guess you rasterized it at 25 m. resolution using polygon to raster tool and the maximum area option. You should check that your raster and the shapefile are aligned. If they don't, then I think that you should do it at a higher resolution if you can (5 or 10 metres would be enough) so the conversion will be more accurate.

- When you use zonal statistics, you may have a problem if some of your buffer zones are mutually overlapping, because ArcMap rasterizes the zones to perform the calculation. I don't know why, but with the old Arcview 3.2, the overlapping was not a problem.

Hope it helps,

Jose

Hi Joao,

Since you have the coordinates of the points around which you want to estimate % cover of each class you could do the following:

Thank you so much, Varun, that was really helpful!

I can't dissolve the classes, though. QGIS freezes and then creates an empty layer. Guess I'll have to try my luck with the raster!

You must use the Plugin Statistic QGIS.

This Plugin allows you to extract the numeric statistics from which you can calculate the percentages that interest you

Hi Joao,

Yes it seems like there is a problem with QGIS when dissolving very large vector files. This has been discussed extensively on numerous forums (e.g. https://hub.qgis.org/issues/3828). From my experience there seems to be a problem only when working with a large vector dataset.

As an alternative to dissolving polygons from the QGIS vector menu, you could use other tools within QGIS that are available on more recent versions of the software. You can go to the 'Processing' menu  in QGIS and select 'Toolbox'. A new panel opens up in QGIS from which you can use the GRASS implementation of a number of processing tools. To dissolve polygons use the 'GRASS commands > Vector (v.*) > v.dissolve' tool. Note - you will need to have the GRASS plugin installed in QGIS to use this option.

Thanks for everyone who answered my question, I've finally managed to calculate the percentagens for each region :) hope I can help you guys with something someday!

Could you please share the way that you were dealing with these issues. Thank you!