topic Re: Slope Calculation Problem in ArcGIS Spatial Analyst Questions

Slope Calculation Problem

KenBerglund — Sat, 12 Feb 2011 02:47:01 GMT

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

Re: Slope Calculation Problem

DanPatterson_Retired — Sat, 12 Feb 2011 09:33:43 GMT

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

Re: Slope Calculation Problem

KenBerglund — Sat, 12 Feb 2011 16:44:03 GMT

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?

Re: Slope Calculation Problem

MichaelStead — Sat, 12 Feb 2011 17:14:11 GMT

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.

Re: Slope Calculation Problem

MichaelStead — Mon, 14 Feb 2011 01:20:58 GMT

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.

Re: Slope Calculation Problem

KenBerglund — Mon, 14 Feb 2011 17:15:13 GMT

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.

Re: Slope Calculation Problem

DanPatterson_Retired — Mon, 14 Feb 2011 18:46:02 GMT

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

Re: Slope Calculation Problem

RyanHollamby — Tue, 01 Mar 2011 12:46:45 GMT

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

Re: Slope Calculation Problem

PatrickTaurman — Tue, 01 Mar 2011 13:09:34 GMT

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 14:16:25 GMT

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

Re: Slope Calculation Problem

PatrickTaurman — Wed, 02 Mar 2011 14:32:29 GMT

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z000000v2000000.htm

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 14:46:11 GMT

Reading through that article, I have to wonder if there is something wrong with my Z units. Mine is always 1, not "zFactor = 0.3043" as described. I thought if my x,y,z are in meters that I leave my z value as 1 (?).

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

Thank You!
Ryan

Re: Slope Calculation Problem

PatrickTaurman — Wed, 02 Mar 2011 14:49:21 GMT

One thing you need to take care of is that everything is in the same units. If elevation values (Z) are in meters, but you define the Projection (XY) as feet, you will not get the correct slope. This is where the Z Factor comes into play. Are you positive your Z value are in meters?

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 15:11:18 GMT

One thing you need to take care of is that everything is in the same units. If elevation values (Z) are in meters, but you define the Projection (XY) as feet, you will not get the correct slope. This is where the Z Factor comes into play. Are you positive your Z value are in meters?

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 15:34:31 GMT

These stripes look like artifacts produced by some forms of tiled compression followed by projection. This suggests the problem is inherent in the original DEM. Take a close look at it (hillshade it if you can) to verify, then go back to the source to track down where the corruption occurred. If it's in the source, the best way to eliminate the stripes is by constructing a custom spatial filter from the FFT of the original grid (which, unfortunately, is not something ArcGIS can help you do).

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

Re: Slope Calculation Problem

PatrickTaurman — Wed, 02 Mar 2011 15:43:52 GMT

Bill is correct, you will need to apply a Fast Fourier Transformation to your DEM. There are several articles describing the striping you are seeing (including from the USGS website).

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

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

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 15:55:01 GMT

Bill is correct, you will need to apply a Fast Fourier Transformation to your DEM. There are several articles describing the striping you are seeing (including from the USGS website).

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 16:53:48 GMT

So I went and grabbed some DEM's from other parts of the country that are 'updated' and wouldn't be subjected to these artifacts and got the same dang thing. So that means I am still doing something wrong.

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

Re: Slope Calculation Problem

RyanHollamby — Wed, 02 Mar 2011 22:14:37 GMT

WOW! I finally figured it out!

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

Re: Slope Calculation Problem

LauraShaffer — Thu, 07 Apr 2011 20:56:19 GMT

Was this ever resolved? I'm having the same problem, but I'm still confused about what I'm supposed to do with the nice fresh NED tif I just downloaded from the USGS Seamless Server. I've never had trouble calculating slope etc. from DEM files downloaded elsewhere.

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