Clipping orthophotos

From Bellman
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Brian Wilson 16:04, 16 Sep 2004 (PDT)

Those little suggestions -- it would be cool to have the area outside your park boundary be black and white and the area of interest be in color...

It took me a week to figure out how to do this so I best write it down for next time.

I went through all the steps to learn how to do it wrong in Arc/Info before figuring out the best way in ArcMap. Now I can even do it at home. (I have Spatial Analyst at home but not Arc/Info)

I will add some photos here someday.

My first concept was to create a full color photo in the center of my map by clipping the orthophoto to match the park boundary. To do this, I had to convert the 4 photos in TIFF format into GRID format, then merge them together. The tricky part was realizing that the GRID format splits each photo into 3 bands, and the ESRI tools will only process one band at a time. If you decide to go this route, after processing each band separately you can glue them back together using "MAKESTACK" in either Spatial Analyst or Arc Workstation.

It's a mistake though. When I converted the TIFF to GRID, the IMAGEGRID command gave me 'sharpened' images. It altered the color saturation such that the finished map was ugly. Perhaps there is a way to control this but I took another route.

Instead of clipping the edges off the color orthos, I punched a hole through the b&w image and put the b&w image on top. The result is the same -- b&w surrounding a color center, but I did not have to process as much data (just one band for b&w photos) and did not have to modify the more attractive original color TIFF photos.

To get more information on the relevant ESRI tools, use the search box at http://www.esri.com/ to look for "Practical Guidance for Raster Graphics" and "Mosaic multiband images using ArcGIS and Spatial Analyst".


Summary

Assumption... you have set up a map with hiking trails, features, sign posts, contour lines, things like that. As a backdrop, you want a color ortho photo inside park boundaries and b&w outside.

You have loaded in both the color and b&w photos.

I am using ArcMap 8.3, with the Spatial Analyst and Xtools extensions.


Blow-by-blow

Create a polygon layer with a park-shaped hole in the middle. I happen to have a copy of Xtools. I will try to figure out how to do this without it tomorrow, I promise. The original reason that I created this layer was to put darkness around the park; you can do this and stop here if you want. It looks pretty good, too. To get this effect all you have to do symbolize the layer as black and then set display transparency to some value greater than 0%.

Anyway. We're up to fancier things here.

I have a polygon layer with park boundaries in it. I selected the park that I am interested in, and then made it into a layer. RIght click on the layer and then do "Selection->Create Layer from Selected Features".

Next in ArcCatalog I created a new polygon shapefile and added it to my map. Using the Editor in ArcMap, I zoomed out so that I could see the park and a goodly area around it. (I didn't want the outside borders showing up in my finished map. Then I created a roughly square polygon that would cover the whole park, and saved it and "stopped editing". You have to issue the Stop Editing command before running Xtools on the new polygon. To erase the park area from this polygon, I used "Tools->Xtools->Layer Operations->Erase Features".

At this point I have a shapefile called ParkOutside and can use it as described above. Symbolize black, display say 40%.

Next you have to convert your polygon shapefile into a raster file so that it can be used as an "analysis mask" in the merge operation.

Next I create a GRID of each of the required B&W orthophotos. After figuring out which photos cover my eventual map completely. The easiest way to do this is to right click the name of the photo in ArcCatalog and select "Export->Raster to Different Format". Then you can convert the file and save it in ESRI GRID format in one step. Repeat for each photo.

Now you have a collection of grids and can use the Spatial Analyst Raster Calculator to merge them together. This is where the tricky part comes in.

To merge the photos and cut the hole for the park.