Slope Calculation Problem

[KenBerglund]

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

[DanPatterson_Retired]

The only time you define the projection of a dataset is when it doesn't have one.  You don't define it to what you want it to be, you must project it.  If your data are indeed in decimal degrees, then you need to project it to a planar coordinate system such as UTM etc etc.  When that is done, ensure that your Z values are the same as your X, Y units (eg meters) or apply the appropriate correction (ie feet to meters etc) before calculating slope

[KenBerglund]

Yeah, I tried to project to UTM but there was an error. And I think it is due to "linear units" not being defined in the DEM. Would that cause an error? And how do you define the linear units?

[MichaelStead]

Looking at the USGS website it seems they distribute data in utm and geographic. I  would try removing the projection information you defined and playing with datasets of known geographic and utm projections and letting the dataframe projection shift the DEM and the data around to give you an indication of which flavour you have before attempting to define the projection.  I don't think the distinction between defining a projection and projecting data is the problem here, but it is surprising how many times this is not obvious to users.

[MichaelStead]

I was just thinking that somehow a projected system (granted with a geographic name) is being defined for a geographic data set and then a projection is being attempted to UTM and that is what is causing it to fail.

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.

[KenBerglund]

Hi everyone,
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.

[DanPatterson_Retired]

the file has x/y coordinates of decimal degrees (aka unprojected, aka decimal degree data), it has a NAD 83 data, if it wasn't defined, then you can define it as a GCS NAD83 (in the unprojected coordinate system, north american datum section).  The Z values will either be in feet (possible if in the US) or meters (possible in the US but almost guaranteed in the rest of the world...some countries and legacy datasets excepted).  Once you have a defined coordinate system using the Define Projection tool, you then...and only then...may proceed to project it using the Project tools (either within the raster or vector section of the Data management tool, Projections and coordinate systems...yadditty, yaddity, yaddity...depending upon your version.

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

[RyanHollamby]

Did you ever figure out a solution to this? It is the exact problem that I am having and can't fix it.

Any help would be great.

Ryan

[PatrickTaurman]

Hi 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

[RyanHollamby]

Thanks for your response!

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

Hi 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

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