[gdal-dev] Slope obtained with GDAL has weird lines

Sat Mar 21 16:57:32 PDT 2020

Thanks for your answers Martin and David.

Yes, you are right, I will use an other data source without that problem.
An other user sent me an article about *Release of NASADEM Data Products*
at *https://lpdaac.usgs.gov/news/release-nasadem-data-products/.*
<https://lpdaac.usgs.gov/news/release-nasadem-data-products/>

I think that using this data would be the best option.

Thanks again!
Danilo

El mar., 17 mar. 2020 a las 19:50, Martin Chapman (<
mchapman at hyperacktive.com>) escribió:

> Danilo,
>
>
>
> Resampling only occurs when an image is being resized or warped.  If the
> dimensions are not changing then resampling is not really appropriate.  If
> you do need to resample, the utilities called gdal_translate.exe and
> gdalwarp.exe may be used depending on your needs.  Other utilities also
> provide resample options but they may not fit your purposes.
>
>
>
> As a general rule you should NOT use nearest neighbor to resample
> elevation (floating point) data because it can drastically alter the
> elevation values from the original values and creates a staircase like
> effect in the output imagery.  Use something like cubic or bilinear.  That
> said, if the input data is bad then nothing you do will fix it.  Maybe look
> on the web for the USGS 3Dep elevation data if you are in the USA or SRTM 1
> arc second data for continuous worldwide coverage.  Maybe the source data
> you are using is just bad or has been altered.
>
>
>
> You should definitely NOT resample the slope output.  I think your source
> data may just be bad but it’s hard to know.  Download other sources and
> compare the output.  If the output looks similar then review your code or
> command line options.
>
>
>
> Best regards,
>
> Martin Chapman
>
>
>
> *From:* gdal-dev [mailto:gdal-dev-bounces at lists.osgeo.org] *On Behalf Of *Danilo
> da Rosa
> *Sent:* Tuesday, March 17, 2020 2:25 PM
> *To:* gdal-dev at lists.osgeo.org
> *Subject:* Re: [gdal-dev] Slope obtained with GDAL has weird lines
>
>
>
> Thanks for your answers Joaquim, Alex and Martin!
>
>
>
> I think the problem is height is measured by 1m intervals, thatś why I see
> 1 meter contour lines in the slope result. The dem grid was not
> interpolated from contours, at least not by me, the data comes from Mapzen
> terrain tiles, I don't know exactly how they are producing it.
>
>
>
> Do you think it would be a good idea to do some kind of interpolation to
> smooth the DEM file or the slope file? Do you have any recommendations on
> how to do that using gdal? The idea is to use the gdaldem color-relief
> command to generate a coloured and easy to read map. The problem is that
> this lines makes the map more difficult to understand, which is a priority
> in this case.
>
>
>
> Thanks again!
>
> Danilo
>
>
>
>
>
> El mar., 17 mar. 2020 a las 4:50, Martin Chapman (<
> mchapman at hyperacktive.com>) escribió:
>
> A slope map indicates the maximum rate of change between a pixel elevation
> value and its neighboring 8 pixels.  In other words, the output value for
> each pixel represents the steepest slope, or decent to the neighboring
> elevation values.  The output values are given as either percent slope or
> degrees from 0 (flat) to 90 (vertical) depending on the command line
> switches you choose.
>
>
>
> Therefore, the output raster values no longer represent elevation values
> from the input but rather slope (steepness) values and should be
> interpreted differently.  A common way to visually interpret them would be
> to color code them with a color ramp where each color in the ramp
> represents a range of degrees somewhere between 0 and 90.  That way a user
> can look at the image and quickly identify steep slopes versus not so steep
> slopes and flat areas.  To create a color ramp that matches changes in
> steepness you will need to use a statistical method on the slope values
> such as standard deviations or jenks natural breaks to group similar slope
> values together and where they change.  In other words, you will want your
> color ramp to change colors where flat areas start to get steeper.  I hope
> that makes sense.
>
>
>
> Another way to use the results would be to write an algorithm to identify
> areas that are within a certain range of steepness so you could identify a
> good place to build a road or perhaps lay train tracks that must not exceed
> a certain steepness.
>
>
>
> I hope that helps.
>
>
>
> Best regards,
>
> Martin Chapman
>
>
>
> On Mar 16, 2020, at 5:09 PM, Danilo da Rosa <daniloide at gmail.com> wrote:
>
> 
>
> Hi!
>
>
>
> This may be a very basic question, but I can't find the answer anyware.
>
> I am working with this DEM obtained using terraincache
> <https://pypi.org/project/terraincache/>, it uses mapzen data. The SRC is
> WGS 84.
>
> [image: DEM File in QGIS] <https://i.stack.imgur.com/Sp4Sg.jpg>
>
> I ran this GDAL command to get the slope:
>
> gdaldem slope -s 111120 dem.tif slope.tif (I had to do scaling because of
> the SRC)
>
> The resulting image looks fine but it's not smooth, it has weird lines
> over it, like contour lines.
>
> [image: Slope file on QGIS] <https://i.stack.imgur.com/POcHR.jpg>
>
> You can download the DEM file here: https://gofile.io/?c=Du7WnM
>
> Does anyone have an idea of how to solve it?
>
> Thanks in advance for your help,
>
> Danilo
>
> P.S. I made this question here too:
> https://gis.stackexchange.com/questions/353983/slope-obtained-with-gdal-has-weird-lines
>
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