How to combine raster (with daily precipitaition data) to recieve monthly values

[HelenaAsttrom]

Hey,

I'm very new to ArcGIS and I have run into the following problem. I have 30 raster files where every pixel has a certain precipitation value (TRMM). I would need to calculate the total precipitation for a certain area of the raster, i.e. I could need to find a way to calculate the values for each pixel and combine them to one raster that shows the total value. How can I do this?

I tried with combining the raster layers (spatial analyst-local-combine) but it doesnt give a correct answer.

I hope someone of you can help me out! thanks!

[DanPatterson_Retired]

add them (aka... plus)

[TimothyHales]

Hi Helena,

Dan is right, you will need to use the Plus tool.  This tool is found under the Spatial Analyst Toolbox > Math.  You can only add two raster at a time with the Plus tool.  You can however use the Raster Calculator (Spatial Analyst > Map Algebra) and create an expression to add all of the rasters. 

-Timothy