topic Deleting excess outer map by using a shapefile as a polygon in Data Management Questions

Deleting excess outer map by using a shapefile as a polygon

JamesGriffiths — Tue, 09 Aug 2011 14:50:04 GMT

Hi, i'll try and make this as easy to understand for my first post.

I'm using ArcMap 9.2.

Ok, I've created a NDVI raster for a large map I have so the date ranges from -1 -> 1.
I also have a shapefile as a seperate layer.

What i'm trying to do is use the shapefile as a mask to cut away the excess map that I don't need.

I can do this using the extraction tool (spatial analyst -> extraction -> extract by mask), the clip tool (data management -> raster -> clip) or by using the analysis mask under the spatial analyst toolbar analyst mask (spatial analyst -> options -> general) and then evaluating my NDVI layer in the raster calculator).

However, my problem is that when I complete this on my NDVI layer, the remaining, useful, output raster has data ranging from -0.3 -> 0.6 (not the original -1 -> 1).

How do i stop the data changing as such so that it remains as the same data range as the orginal layer.

Am I simply not ticking the correct box somewhere along the line?

Your help would be appreciated, thanks!

Re: Deleting excess outer map by using a shapefile as a polygon

MarkEllis — Thu, 11 Aug 2011 19:27:08 GMT

Is there any chance that the values -1 to -0.4 and 0.7+ only lie within the area you have masked?

Re: Deleting excess outer map by using a shapefile as a polygon

TimothyTresohlavy — Fri, 02 Sep 2011 14:32:47 GMT

Rather than deleting, would it be easier to just cover these areas? Or are you interested in removing the excess data and saving refresh time?

Under Data Frame Properties > Data Frame > Extent Used by Full Extent Command > Other: > Specify Extent > Outline of Features > Layer: [select mask shapefile]. This essentially tells ArcMap "Don't show me anything beyond the limits of this shapefile", however it does not modify any of your original datasets. Much like a Definition Query.