how do I assign new values to existing raster cells

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06-04-2010 09:22 AM
TedR
by
New Contributor II
Hi,

I am trying to assign new values to an existing raster dataset and have tried several approaches, to no avail.  What is the simplest method to do this?  My raster dataset cells contains 120 unique values (0 to 119), and I'd like to change each of these.   Ex.  Cells with 0 will be changed to 0.75, 2 = 0.18, 11 = 0.1, 61 = 1.82, 97 = 1.93, etc.  The old values that I do not change I'd like to change to 0 or null.

Thanks in advance!

TC
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3 Replies
TedR
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New Contributor II
Thanks whuber.  I was out of town yesterday.  I have a quick follow up question. 

I have attached a screen capture of my attribute table from a shapefile (points) named rastert_geo_stp1_spatialjoin.  Notice that it contains a  "Formation" field and a "Co_Name" (county name) field.  What I am trying to do is find a way to set the combination of those two fields to correspond to a "Z" value of my choosing.  For example, if Formation = granite and Co_Name = Ashe County then that raster point would be set to a value of say 0.3.  Granite in Henderson County would be set a value of 0.7.  And so on.  The goal is to end up with a raster that has my assigned values based on various combinations of Formation and Co_Name...I will then use the raster to depict Z concentrations across N.C.

Background.  I started off with a map of geologic formations (polygons) and a map of counties (polygons).  I am trying to assign a median radon value (my "Z") for overlapping areas from these two maps.  In other words, a formation called granite in county A will have a radon value of X, and granite in county B will have a radon value of X2, etc.   I'm not clear how best "to get from here to there".  

Ted
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TedR
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New Contributor II
Thanks whuber.  I know you are busy and I appreciate your input!

So 2 steps.  First I overlay my two polygon maps (formations and counties) to get new a new polygon map with new polygon zones.  You're saying that the intersection operation is the way to go for this. 

Second, I compute statistics on my "Z" values (radon) that fall within these newly created zones and assign this statistic (preferably the median of all "Z's" in that polygon) to all cells within the zone.   You're saying a summerize operation is the way to go for this.  I guess you mean zonal stats as a table - that's all I could find.  So assuming I use as my input the intersection feature class, what would I use for the "zone field" which is the field that holds the values that define each zone (TWO values define each zone - formation and county, so I'm not sure what to use).

Then somehow I generate a raster of this to render the resulting dataset.
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TedR
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New Contributor II
Many thanks.  I think I know how to proceed.  I appreciate your help.  This is my first post so I'm not sure how to formally acknowledge your good assistance (with points?).  If I can figure out how to do that I will.

Best regards.

Ted
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