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Composite CN Using NLCD TIFF Data (Follow-up)


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On July 18th I posted a request for information related to computation of composite Curve Numbers with WMS

using NLCD images. I received a response that WMS version 9.0 offers the ability to download 2006 NLCD

data using the Get Data tools and it reads the data as a land use grid.

I just tried that feature on a modeling project that I recently completed. I had previously used GIRAS

based LULC shapefile data on that project with disappointing Curve Number results. I had to modify the

curve number using aerial photography of the relatively small watershed in conjunction with SSURGO polygons

displayed in WMS, and a spreadsheet that computes composite CN's based on percentages. I felt there had to

be a better way, and I learned that this task could be done with NLCD data. That prompted me to post my

question on July 18th.

I was pleased to see that the 2006 NLCD download with the Get Data From Map tool worked flawlessly and I

proceeded to generate a CN from WMS very quickly. However, once again the CN computed by WMS was not

appropriate based on the land use and land cover characteristics of the watershed. The polygon based CN

was too high, and the 2006 NLCD raster based CN was way too low. I believe the problem is that land

use/land cover data resolution of the 2006 NLCD data is poor in the project area, which is located in

southwestern Pennsylvania.

I have learned that there is a different raster dataset circa 2005, developed by Penn State University for

the entire state of PA, and funded by the state's Department of Conservation and Natural Resources (DCNR).

This data would offer an alternative to 2006 NLCD in situations like this. The raster is in the form of a

large TIFF file. Based on a little research, I believe this kind of land use/land cover TIFF can be

converted into a land use grid, but I do not know how to do this. I have been doing hydrology and

hydraulics modeling for a long time, but I am a new user of GIS technology, with WMS being the only GIS

software that I am somewhat acquainted with.

Is there a way that I can convert this large TIFF into a land use grid in WMS, or is a different piece of

software required to accomplish the task?

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I think you can convert this land use grid into something you can use in WMS. You have a couple of options for reading the file into WMS. The first option is to get the grid into a format called "Arc/Info ASCII grid" format. If you can get your grid into this format and change the file extension to ".lu" or something that WMS does not immediately recognize, you will be prompted for the file type. Select the Land Use Grid (*.*) option and WMS will read this file as a land use grid and you're done.

Another option you have is that if you have ArcMap 10.0 or later on your computer, you can enable ArcObjects within WMS and read the land use grid in any format ArcObjects supports. There is then a right-click option on the object to convert the grid to a land use grid that can be used to compute CN in WMS.

In a future version of WMS, we will have an option to read the .TIF file directly and convert it to a land use grid with all the palette attributes defined in the land use attributes. WMS 9.0 just thinks a .tif file is a regular image and there is no way to convert it to a land use grid that can be used to compute CN.

Let me know if you have any questions about this.


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