Hello,

I am having problems simply calculating % slope from a USGSDEM. Even though it had a GCS_North_American_1983 projection, I defined its projection to be that as well after reading some posts on the same issue. The resultant slopes have values into the millions. I then tried to project the DEM into UTM to ensure the units would be in meters. This caused an error. Under the spatial reference in the DEM, the "Linear Units" are blank. This is why I think the project to UTM failed as it does not have linear units defined. Anyone have any ideas? This should have been a simple task but has been troublesome. I guess I am asking, why would the slope % be in the millions? Or any ideas on why the project to UTM failed? Or how to define linear units?

Thanks for any help

Ken

I am having problems simply calculating % slope from a USGSDEM. Even though it had a GCS_North_American_1983 projection, I defined its projection to be that as well after reading some posts on the same issue. The resultant slopes have values into the millions. I then tried to project the DEM into UTM to ensure the units would be in meters. This caused an error. Under the spatial reference in the DEM, the "Linear Units" are blank. This is why I think the project to UTM failed as it does not have linear units defined. Anyone have any ideas? This should have been a simple task but has been troublesome. I guess I am asking, why would the slope % be in the millions? Or any ideas on why the project to UTM failed? Or how to define linear units?

Thanks for any help

Ken

Being that it is raster data and the specifics of the error are not given there are probably a lot of possibilities unrelated to projections that could be causing problems in the projection.

Yes, the problem is the coordinates being in decimal degrees. I only defined the projection because of a related problem I read in the forums. I know that you don't need to define a projection once it has been projected already. So my problem is to get the DEM out of decimal degrees, which is what I have been trying to do. But when I try to get the units to meters by projecting to UTM, the projection to UTM fails. And I think this is because in the DEM, the linear units are not defined (look at attachment at initial post). So how do you get the linear units defined (not sure define is the right word)?

Thanks for any help.

I yearn for the days where you couldn't use a grid or shapefile unless it had a coordinate system defined...projection on the fly and weak warnings about coordinate systems not being defined, produce a lot of traffic on this forum, confusion amongst users (of many levels) and should be rectified within the software...rant over...give it a try and document your exact steps should any of the above fail to produce a dataset from which you can determine a slope grid. Good luck

Any help would be great.

Ryan

Is your raster in a geographic or projected coordinate system? If geographic, please re-project to a projected coordinate system using the Project tool, then run the Slope. If your raster is currently in a projected coordinate system please see the following Web Help:

The range of slope values in degrees is 0 to 90. For percent rise, the range is 0 for near infinity. A flat surface is 0 percent, a 45 degree surface is 100 percent, and as the surface becomes more vertical, the percent rise becomes increasingly larger.

http://help.arcgis.com/en/arcgisdesk...00v2000000.htm

I can't get that link to work, maybe it is on my end. Do you have titel?

Below is what posted this in another section, sorry to be reduntent:

My raster (DEM) that I am getting is from http://viewer.nationalmap.gov/viewer/

Here is the metadata: http://extract.cr.usgs.gov/distmeta/...&DATASET=NED13

I started a new .mxd to make sure to try and clear any projection changes/definations that I have played with. Loaded the DEM which I download from above. It allows me to choose: the DEM as a GeoTiff, ArcGrid, Grid Float, or a BIL_16INT. I went with ArcGrid, after a little search (maybe I was incorrect to go this route).

So the meta data states:

"All NED data are distributed in geographic coordinates in units of decimal degrees, and in conformance with the North American Datum of 1983 (NAD 83). All elevation values are provided in units of meters, and are referenced to the North American Vertical Datum of 1988 (NAVD 88) over the conterminous United States"

Once I uploaded the ArcGrid into ArcInfo, I checked the projection of the data frame and data and it is what it states above in bold. So I reprojected it to UTM State Planer 13N (for Colorado), because I think I need it in meters instead of decimal degrees (right?). I used the Data Management Tools-->Projections & Transformations-->Raster-->Project Raster to take it from the geographic coordinates to the state planar (again, hoping I used the rigt toolbox tool).

Then I did 3d Analyst--> Raster Surface--> Slope.

So after this, now my values are reasonable (0-100% or 1-90 degrees) in the Table of Contents (unlike 5000 from before), but I am working with a area with lots of mountains, and only receiving two different values on the image. So obviously I still have something projected wrong and/or going about this incorrectly.

Any help or insight you could provide would be great. I'm assuming that I am missing something very obvious. I am going to keep searching around, and reading through the help and trying different combinations.

I appreciate everyone's time, thank you!

Ryan

Also I am curious about your answer "now my values are reasonable (0-100% or 1-90 degrees)". 45 degrees = 100% slope. So if you are getting an answer of greater than 45 degrees, your slope percent has to be greater than 100%.

As for the reprojecting, you used the correct workflow (Project Raster tool) to go from NAD 1983 to State Plane.

You may want to contact Support so someone can take a closer look at everything.

Patrick

Good point about the degree problem. I will continue you looking and trying.

Thank You!

Ryan

Both XY & Z are in meters.

Attached is what is looks like now. The values are correct, the red is steeper terrian and that is what the terrian looks like in the field, but has the grid within it which is ruining the overall projection. The values in the Table of Contents is between 0-90.

Seriously thankyou

## Attachments

Hmm...I am going to try and select several other of the formats from the USGS's nationalviewer website and see if I can correct that way. I'll see if the other formats that I can download work

The hillshade also have the artifacts embedded.

Thank you!

Ryan

http://ned.usgs.gov/Ned/about.asp

http://seamless.usgs.gov/faq/ned_faq.php#seven

http://www.ctmap.com/assets/pdfprojects/destripe.pdf

Well all of those links have images exactly how mine are turning out. I am working with the Pikes Peak region, so I am surprised that it falls along with "In areas where the only available DEM is produced by older methods, then "striping" may still occur."

Is there someplace else that I should be getting my DEM from? Seems strange to have this many issues just from downloading a DEM from USGS and getting the slope values.

When I have the raster in my data set, before performing the slope stuff, there is no artifacts and very clear.

Thanks again,

Ryan

I appreciate everyones help. So I am going to run through a bunch of different ways and try to figure out what the heck is going on.

Thanks

So after you set me straight on defining the projection and projecting the raster, getting the x,y, and z to meters, I figured out the missing piece.

The data from the USGS was fine as a ArcGrid. The link I was missing was when Projecting the Raster. The artifacts that I was generating was from the resampling techinque. I didn't know the difference between nearest, bilinear, cubic or majority. Since it was optional I brushed it off (MISTAKE). As I worked back through each step...researching each step I came across the meaning of the techniques. Went back in to the toolbox and then read the little help description. For the reasons in bold I went with Bilinear.

From the help:

"The NEAREST option, which performs a nearest neighbor assignment, is the fastest of the four interpolation methods. It is primarily used for categorical data, such as a land-use classification, because it will not change the cell values.

Do not use NEAREST for continuous data such as elevation surfaces.The BILINEAR option, bilinear interpolation, determines the new value of a cell based on a weighted distance average of surrounding cells. The CUBIC option, cubic convolution, determines the new cell value by fitting a smooth curve through the surrounding points. These are most appropriate for continuous data and may cause some smoothing; also, cubic convolution

may result in the output raster containing values outside the range of the input raster. It is not recommended that BILINEAR or CUBIC be used with categorical data because the cell values may be altered. "Anyways now I have a perfect slope file!!

I appreciate you helping me through this. Little did I know I had 3 big mistakes. I think that this sample technique is a common contributor to people's problems (seems how I read every single slope thread in this forum). The people that state the projection is set, values are the same (meters, feet, etc) yet they still get grids.

This thread is where I started: http://forums.arcgis.com/threads/24718-Slope-Values-Projection-Issue?p=82179#post82179

Thanks again. I hope someone that is having this problem tries this as a solution.

Ryan

Without doing anything to the NED, ArcCatalog says that the Spatial Reference for it is GCS_North_American_1983. Is that the same as the projection? When I downloaded it, USGS specified the output format as "ArcGRID NAD 83 Geographic." What am I supposed to project it to? Everything else in my map is in NAD_1983_UTM_Zone_12N (it's northern Arizona).

I understand that the horizontal coordinates are probably in decimal degrees and the vertical are probably in meters. I'm not sure how to convert these to something usable (and I'm not sure where to look to confirm the units on these).

I tried projecting the NED to the 12N projection, then doing a raster calculation to convert the elevation (z) from meters to feet, then tried getting slopes (via both 3D and Spatial Analyst, is there a difference?) but the % slopes were still in the thousands. After all that I downloaded a fresh NED in case I messed up the original in the process somehow (superstitious, maybe but this is driving me crazy!).

So . . . how do I get from an NED from USGS to slope? Thanks in advance!!

-Laura

I realize that this is an �??old strand�?� �?? maybe even ancient (two years!) �?? but in case if anyone�??s still listening: With respect to the Slope Tool in ArcGIS 10.1, I�??ve tried the fix that Randy discovered and it hasn�??t worked for me.

That is, (i) my raster data is projected and has linear units (USGSDEMS having spatial reference: NAD_1983_UTMzone_14N, Linear Unit: meter (1.0), Angular Unit: degree (.0174), Datum: North_Am_1983), (ii) the z and x,y units are the same; and (iii) I applied the �??bilinear interpolation�?� resampling option -- BUT I�??m still getting wonky slope values, i.e., the smallest range category is 0 �?? 319,000 or so. Is it possible that degrees are still being used in the algorithm for some reason? Also, �??resampling during display�?� seems to have no effect �?? might there be something I�??m not doing and should be doing?

By-the-by, does anyone know why using an Output Measurement of �??Per Cent Rise�?� at least result in a recognizably correct image, whereas using �??Degree�?� does not, at least not for me

Thanks for any help that anyone has time to give.

Blair S.