I am trying to match rainfall data from USGS/FEWS to coordinate locations of survey households in Ethiopia, so I will need to be able to project the rainfall data. So far, I downloaded the dekadal RFE 2.0 files for East Africa (bulk by year), and for each dekadal, there are five files: .bil, .blw, .clr, .hdr, and .stx. I first added the data (the .bil file) to QGIS, using Africa Albers Equal Area Conic as Projected Coordinate Systems. And then, I opened CSV data of village latitude and longitude and saved it to Africa Albers Equal Area Conic.They looked fine and I could extract point values using point sampling tool.
However, there was wide dispersion from the precipitation data provided by survey. The survey uses the same USGS/FEWS rainfall data as I did, but uses household location rather than village location information. Since dispersion is so wide, I suspect that dispersion came from CRS I chose, rather than household/village level difference. From the USGS/FEWS website, I know following projection information:
Coordinates for corners: Lower left lat : -42.243 deg Lower left lon : -23.490 deg Upper left lat : 43.711 deg Upper left lon : -24.600 deg Lower right lat : -42.242 deg Lower right lon : 63.414 deg Upper right lat : 43.712 deg Upper right lon : 64.523 deg Image size : 1152 rows x 1152 cols Center lat,lon : 1.000000, 20.000000 Pixel size h x w : 8.000000 km x 8.000000 km Origin of latitudes : 1.000000 deg Central meridian : 20.000000 deg First std parallel : -19.000000 deg Second std parallel : 21.000000 deg Projection = ALBERS Conical Equal-area projection uses the clarke 1866 spheroid
Using this information, I am trying to custom projection as follows:
+proj=aea +lat_1=-19 +lat_2=21 +lat_0=1 +lon_0=20 +x_0=0 +y_0=0 +datum=NAD83 +units=m +no_defs +ellps=clrk66
Can anybody confirm whether this projection is correct? I am beginner of QGIS user and only know the following basic parameters.
- lat_1 : First std parallel
- lat_2 : 2nd std paralell
- lat_0 : Origin of latitudes
- lon_0 : Central meridian
But I am not sure about +datum and +units.
@Spacedman, thank you very much for exploring actual data. I hope what I did is fine.
I re-did all the process using projection I made above:
- Add dekadal rainfall data as raster layer using projection I customed.
- Add Ethiopia administrative data as vector layer. I downloaded it from the website of Global Administrative Areas. This layer appeared as a small point on QGIS screen because Ethiopia administrative data is provided as WGS84 projection.
- Save Ethiopia administrative data to the projection I customed.
- Add village longitude and latitude csv data as delimited text layer using WGS84.
- Save village longitude and latitude data as ESRI shapefile to the projection I customed.
- Extract rainfall data using point sampling tool. Save the newly created layer as csv.
And I got exactly the same attribute table I got with Africa Albers Equal Area Conic projection. So, I do not know why the rainfall data I extracted do not match the survey rainfall data. Is my entire procedure correct?
Below is the table I made with survey data and extracted rainfall data; h2011_tot (annual total rainfall of 2011 from the survey) and total2011 (sum of all the 36 dekadal data I extracted as above). The survey data contain 2011's annual total rainfall as well as village level longitude and latitude information which I used above. I wanted to produce 2012's annual total rainfall but I first made 2011's annual total rainfall to confirm it matches with survey rainfall data. And I found they do not match each other.
As we can see below, there is wide dispersion between h2011_tot and total2011, although their means and standard deviations seem very close to each other. Do you have any idea why h2011_tot and total2011 look so different?