Clip 2 Grids together with NoData regions

Clip 2 Grids together with NoData regions

Post by Artur Grall » Tue, 05 Dec 2000 04:00:00



Hi All,

I have created 2 Topogrids with ArcInfo 8. The data in Grid1 has some
regions with NoData and in this NoData regions I have to "import" (or
merge, or mosaic???) the  Grid2 (just some islands of data).
The problem is to join the data in ArcView 3.2, because the Output Grid3
is always completely NoData. I used Analysis/MapCalculator and then
"Grid1+Grid2".
I guess, if NoData should be joined with data, the result is NoData.

Any Ideas to get correct results?

Best regards from Hamburg,
Artur

 
 
 

Clip 2 Grids together with NoData regions

Post by mapme.. » Fri, 08 Dec 2000 04:00:00


Artur,

When you add two grids together NODATA always overwrites data.

The way around this is to convert the NODATA values to a value.
You can do this in ARCINFO using the ISNULL function in a con statement
eg
newgrid1 = con(grid1(isnull), 9999, grid1)

this will convert all the NODATA values to 9999.  Do the same with grid2
and then add them.

Hope this helps

Craig

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1. If only VueScan could identify clipped regions...

There's just one more thing I wish I could do with VueScan - or perhaps it's
there, and I've missed it...

One of the really cool things that PhotoShop lets you do with the Levels
command, is to move the input sliders, while pressing Alt/Option. This lets
you see just what will be lost if you clip the levels at any particular
point.

I know that in principal, I can look at the numbers displayed in the info
bar as I move the cursor over bits that I think might be affected by a
levels tweak - but the way PhotoShop does it is a lot more intuitive.

I've been surprised by the number of times that I've looked at the preview
histogram, and decided that there was nothing of consequence in the
highlights or shadows - only to discover, too late, that there was an area
that wasn't very big, but it was important enough to preserve the details in
it...

I don't know if VueScan adjusts the exposure in my Canon FS4000US to
optimise the dynamic range that I set with the black & white clipping
levels - or if I might as well leave them at 0% clipping, and tweak the
high-bit file in PhotoShop.

Cheers,

John

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