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Raster creation help

TimJackson

I apologise if this has been answered before, as it seem like quite a basic question but I couldnt find a solution. (9.3, ArcView license with spatial analyst and 3D analyst).

I am trying to calculate the volume of water that would be impounded by a hypothetical dam using cut/fill, and I am having trouble creating the "after" raster.

I have an elevation raster (Ras1) and a polygon of the surface of the potential reservoir formed by the dam and a 450m contour.
I used "extract by mask" to create a raster of the reservoir and reclassified this to a constant 450m (Ras2).
I am now trying to create a raster (Ras3) that is identical to Ras1 apart from the area covered by Ras2, which should take the Ras2 values (450)

I have tried using raster calculator an con([Ras1] > 0, [Ras1], [Ras2]) but the output is identical to Ras2, with no data from Ras1.

Where am I going wrong with this? and is this even a sensible method?

Once I have Ras3 I am assuming cut/fill with Ras1 as "before" and Ras3 as "after" will give me the volume

I have found the volume using TIN polygon volume already but I want to compare it with the cut/fill method

Thanks
Tim

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Accepted answers

XanderBakker

I have an elevation raster (Ras1) and a polygon of the surface of the potential reservoir formed by the dam and a 450m contour.
I used "extract by mask" to create a raster of the reservoir and reclassified this to a constant 450m (Ras2).
I am now trying to create a raster (Ras3) that is identical to Ras1 apart from the area covered by Ras2, which should take the Ras2 values (450)

I have tried using raster calculator an con([Ras1] > 0, [Ras1], [Ras2]) but the output is identical to Ras2, with no data from Ras1.

Where am I going wrong with this? and is this even a sensible method?

Hi Tim,

When using a formula in Spatial Analyst, you should take special care about the NoData values. Normally combining a value with a NoData value results in NoData.

To avoid this try the following formula:

Con ( IsNull ( [Ras2] ) , [Ras1] , [Ras2] )

In this case it detects the NoData values and returns the values from [Ras1], other areas will be filled with the data from [Ras2].

Further reading in 9.3:
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Is_Null
http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=NoData_and_how_it_affects_analysis

Kind regards,

Xander

All comments

XanderBakker

I have an elevation raster (Ras1) and a polygon of the surface of the potential reservoir formed by the dam and a 450m contour.
I used "extract by mask" to create a raster of the reservoir and reclassified this to a constant 450m (Ras2).
I am now trying to create a raster (Ras3) that is identical to Ras1 apart from the area covered by Ras2, which should take the Ras2 values (450)

I have tried using raster calculator an con([Ras1] > 0, [Ras1], [Ras2]) but the output is identical to Ras2, with no data from Ras1.

Where am I going wrong with this? and is this even a sensible method?

Con ( IsNull ( [Ras2] ) , [Ras1] , [Ras2] )

TimJackson

Thank you Xander, this worked perfectly
Tim

curtvprice

10.x help links:

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