Question on zonal statistics

[AnLat]

Hello,

I am just starting working with GIS and I have two questions. I have a raster file on precipiation and I want to combine it with a shapefile with the boundaries of countries all over the world (e.g. World Bank) in order to get zonal statistics
If  I understand correctly I need to choose the rig ht projection (e.g. Cylindrical, Eckert IV, Eckert VI, Mollweide, Flat Polar Quartic, Sinusoidal) in order to make sure that each cell respresents the same area.
1) Does this indeed resolve the issue? When I tried another projection (GCS_ETRS_1989) I got the same area for all the countries and the same mean for precipitation.

2) While in my original shapefile I have 253 countries, when I do the zonal statistics I am left with 183 coutnries. Any explanation about why this is happening?

Thanks in advance!

[PrasantaBhattarai]

2) While in my original shapefile I have 253 countries, when I do the zonal statistics I am left with 183 coutnries. Any explanation about why this is happening?
!

Hello,

This probably is happening as the processing cell size is not small enough to cover each polygons (Countries). Please see this from the help documentation:

If the zone input is a feature dataset with relatively small features, keep in mind that the resolution of the information needs to be appropriate relative to the resolution of the value raster. If the areas of single features are similar to or smaller than the area of single cells in the value raster, in the feature-to-raster conversion some of these zones may not be represented.

To demonstrate this, try converting the feature dataset to a raster with the appropriate feature-to-raster conversion tool and specify the resolution to be that of the Value raster. The result from this conversion will give an indication about what the default output of the zonal operation will be.

If you have fewer results in the output than you may have expected, you need to determine an appropriate raster resolution that will represent the detail of your feature input, and use this resolution as the Cell Size of the Raster Analysis Settings of the Environment.

Regards,
Prasanta.

[AnLat]

Indeed! This resolves the issue. Thanks a lot!

Hello,

This probably is happening as the processing cell size is not small enough to cover each polygons (Countries). Please see this from the help documentation:

If the zone input is a feature dataset with relatively small features, keep in mind that the resolution of the information needs to be appropriate relative to the resolution of the value raster. If the areas of single features are similar to or smaller than the area of single cells in the value raster, in the feature-to-raster conversion some of these zones may not be represented.

To demonstrate this, try converting the feature dataset to a raster with the appropriate feature-to-raster conversion tool and specify the resolution to be that of the Value raster. The result from this conversion will give an indication about what the default output of the zonal operation will be.

If you have fewer results in the output than you may have expected, you need to determine an appropriate raster resolution that will represent the detail of your feature input, and use this resolution as the Cell Size of the Raster Analysis Settings of the Environment.

Regards,
Prasanta.