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Calculating slope in percentage with QGIS

How to perform a slope analysis in QGIS?How to extract slope from topographic raster map?Creating slope map from DEM using QGIS?Calculating slope between 5 and 10 percent using raster calculator of QGIS?Removing contour artifacts from low slope regionsQGIS slope percent calculationCalculating slope length using LS-toolActual slope between two points from raster slope valueCreating slope map from DEM using QGIS?How to understand/interpret slope unit values in QGIS 2.14?gdaldem slope…odd values, almost all 89.98+/- (? - why?)QGIS slope layer helpcalculate slope of lines between nodes?Calculate slope of line segments with QGISMosaic multiple slope files

2

1

I'm working with QGIS, v. 2.0.1. I have several soil types. They are (amongst others) described with slope indications. I already calculated the slope from the DTM in degrees, but as description of the soil types is in percentage, I need to calculate the slope in percentage.

I tried two things:

1. I tried the analysis tool from the raster section, but it didn't work. The result was just black with values from 0 to 0.


2. The same happened when I tried to calculate the percentage in the raster calculator but the result was the same.

Maybe I need to say that the terrain is pretty plain.
What else can I do?

Update

I tried the raster calculator and did what @radouxju suggested. It gave me a black picture with values from 1756.64 to 1756.64. But as soon as I click in the picture the values are "normal", say 5.88 or 13.224 or something like that. The same happens when I try the gdaldem tool but with values from 0 to 0. I also tried to change the contrast enhancement (like stretch to min/max) but it didn't change anything.

Update 2

I just found out that maybe I have to set the scale to 111120 but that doesn't work neither (result: black, values 0-0).

qgis slope

share|improve this question

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  for a quick change, slope[percent] = tan(slope[degree])*100, or you can change your soil description slope[degree] = atan(slope[percent]/100)
  
  – radouxju
  Jun 6 '14 at 10:49
  

  
  
  
  

add a comment | 

2

1

I'm working with QGIS, v. 2.0.1. I have several soil types. They are (amongst others) described with slope indications. I already calculated the slope from the DTM in degrees, but as description of the soil types is in percentage, I need to calculate the slope in percentage.

I tried two things:

1. I tried the analysis tool from the raster section, but it didn't work. The result was just black with values from 0 to 0.


2. The same happened when I tried to calculate the percentage in the raster calculator but the result was the same.

Maybe I need to say that the terrain is pretty plain.
What else can I do?

Update

I tried the raster calculator and did what @radouxju suggested. It gave me a black picture with values from 1756.64 to 1756.64. But as soon as I click in the picture the values are "normal", say 5.88 or 13.224 or something like that. The same happens when I try the gdaldem tool but with values from 0 to 0. I also tried to change the contrast enhancement (like stretch to min/max) but it didn't change anything.

Update 2

I just found out that maybe I have to set the scale to 111120 but that doesn't work neither (result: black, values 0-0).

qgis slope

share|improve this question

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  for a quick change, slope[percent] = tan(slope[degree])*100, or you can change your soil description slope[degree] = atan(slope[percent]/100)
  
  – radouxju
  Jun 6 '14 at 10:49
  

  
  
  
  

add a comment | 

I'm working with QGIS, v. 2.0.1. I have several soil types. They are (amongst others) described with slope indications. I already calculated the slope from the DTM in degrees, but as description of the soil types is in percentage, I need to calculate the slope in percentage.

I tried two things:

1. I tried the analysis tool from the raster section, but it didn't work. The result was just black with values from 0 to 0.


2. The same happened when I tried to calculate the percentage in the raster calculator but the result was the same.

Maybe I need to say that the terrain is pretty plain.
What else can I do?

Update

I tried the raster calculator and did what @radouxju suggested. It gave me a black picture with values from 1756.64 to 1756.64. But as soon as I click in the picture the values are "normal", say 5.88 or 13.224 or something like that. The same happens when I try the gdaldem tool but with values from 0 to 0. I also tried to change the contrast enhancement (like stretch to min/max) but it didn't change anything.

Update 2

I just found out that maybe I have to set the scale to 111120 but that doesn't work neither (result: black, values 0-0).

qgis slope

share|improve this question

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

share|improve this question

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

share|improve this question

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

edited Jan 10 '16 at 18:33

wittich

2,0681928

edited Jan 10 '16 at 18:33

wittich

2,0681928

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

asked Jun 6 '14 at 10:00

G.H.G.H.

11113

4 Answers
4

active

oldest

votes

7

Use the gdaldem tool http://www.gdal.org/gdaldem.html as

gdaldem slope input.dem output_in_percents.tif -p -of GTiff

From the manual page: "-p : if specified, the slope will be expressed as percent slope. Otherwise, it is expressed as degrees "

QGIS generates a similar gdaldem command and degrees/percents is selected here:

share|improve this answer

edited Jun 6 '14 at 10:13

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

add a comment | 

3

Some other things to notice:

1. If you converted the degree slope to percent by using the tan, check to see that your input and output have floating point values. Your results with values from 0 to 0 seem to me as a integer conversion problem


2. If you are calculating your slope again (using r.slope.aspect or gdaldem), check to see if your horizontal units are the same as your vertical units, that is, check your projection


3. I've seen two ways GIS softwares calculate slope, one based on the maximum change in elevation for each cell in relation to it's neighbor (ArcMAP) and the other based on the harmonic mean of the slopes on the X and Y direction (ERDAS Imagine). Not sure how gdaldem or grass handles. So be aware that your results may vary depending on the tool used.

share|improve this answer

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  GDAL is re-using algorithms from GRASS which according to this document is selecting the steepest slope grass.osgeo.org/grass64/manuals/r.slope.aspect.html
  
  – user30184
  Jun 6 '14 at 11:53
  

  
  
  
  

add a comment | 

1

There are a couple of ways to get a slope raster with units in percent. In addition to the 1st answer below by user30184, you can also choose the GRASS module r.slope.aspect which calculates several geo-morphological layers, including slope, and you can choose to output either in degrees or percent. It's under in Processing menu, under GRASS -> raster.
But I think you need to first check your "pretty plain" DTM. What is the resolution? Check some cell values and be sure you have a slope at all.

share|improve this answer

edited Jun 6 '14 at 10:44

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

add a comment | 

0

How to change the band from 1 to 4. When i tried to convert DSM to slope only 1 band is coming. But i need 4 bands.

share

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

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7

Use the gdaldem tool http://www.gdal.org/gdaldem.html as

gdaldem slope input.dem output_in_percents.tif -p -of GTiff

From the manual page: "-p : if specified, the slope will be expressed as percent slope. Otherwise, it is expressed as degrees "

QGIS generates a similar gdaldem command and degrees/percents is selected here:

share|improve this answer

edited Jun 6 '14 at 10:13

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

add a comment | 

7

Use the gdaldem tool http://www.gdal.org/gdaldem.html as

gdaldem slope input.dem output_in_percents.tif -p -of GTiff

From the manual page: "-p : if specified, the slope will be expressed as percent slope. Otherwise, it is expressed as degrees "

QGIS generates a similar gdaldem command and degrees/percents is selected here:

share|improve this answer

edited Jun 6 '14 at 10:13

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

add a comment | 

Use the gdaldem tool http://www.gdal.org/gdaldem.html as

gdaldem slope input.dem output_in_percents.tif -p -of GTiff

From the manual page: "-p : if specified, the slope will be expressed as percent slope. Otherwise, it is expressed as degrees "

QGIS generates a similar gdaldem command and degrees/percents is selected here:

share|improve this answer

edited Jun 6 '14 at 10:13

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

gdaldem slope input.dem output_in_percents.tif -p -of GTiff

share|improve this answer

edited Jun 6 '14 at 10:13

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

answered Jun 6 '14 at 10:07

user30184user30184

29.2k23057

3

Some other things to notice:

1. If you converted the degree slope to percent by using the tan, check to see that your input and output have floating point values. Your results with values from 0 to 0 seem to me as a integer conversion problem


2. If you are calculating your slope again (using r.slope.aspect or gdaldem), check to see if your horizontal units are the same as your vertical units, that is, check your projection


3. I've seen two ways GIS softwares calculate slope, one based on the maximum change in elevation for each cell in relation to it's neighbor (ArcMAP) and the other based on the harmonic mean of the slopes on the X and Y direction (ERDAS Imagine). Not sure how gdaldem or grass handles. So be aware that your results may vary depending on the tool used.

share|improve this answer

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  GDAL is re-using algorithms from GRASS which according to this document is selecting the steepest slope grass.osgeo.org/grass64/manuals/r.slope.aspect.html
  
  – user30184
  Jun 6 '14 at 11:53
  

  
  
  
  

add a comment | 

3

Some other things to notice:

1. If you converted the degree slope to percent by using the tan, check to see that your input and output have floating point values. Your results with values from 0 to 0 seem to me as a integer conversion problem


2. If you are calculating your slope again (using r.slope.aspect or gdaldem), check to see if your horizontal units are the same as your vertical units, that is, check your projection


3. I've seen two ways GIS softwares calculate slope, one based on the maximum change in elevation for each cell in relation to it's neighbor (ArcMAP) and the other based on the harmonic mean of the slopes on the X and Y direction (ERDAS Imagine). Not sure how gdaldem or grass handles. So be aware that your results may vary depending on the tool used.

share|improve this answer

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  GDAL is re-using algorithms from GRASS which according to this document is selecting the steepest slope grass.osgeo.org/grass64/manuals/r.slope.aspect.html
  
  – user30184
  Jun 6 '14 at 11:53
  

  
  
  
  

add a comment | 

Some other things to notice:

1. If you converted the degree slope to percent by using the tan, check to see that your input and output have floating point values. Your results with values from 0 to 0 seem to me as a integer conversion problem


2. If you are calculating your slope again (using r.slope.aspect or gdaldem), check to see if your horizontal units are the same as your vertical units, that is, check your projection


3. I've seen two ways GIS softwares calculate slope, one based on the maximum change in elevation for each cell in relation to it's neighbor (ArcMAP) and the other based on the harmonic mean of the slopes on the X and Y direction (ERDAS Imagine). Not sure how gdaldem or grass handles. So be aware that your results may vary depending on the tool used.

share|improve this answer

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

share|improve this answer

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

answered Jun 6 '14 at 11:34

DanielDaniel

1,370725

1

There are a couple of ways to get a slope raster with units in percent. In addition to the 1st answer below by user30184, you can also choose the GRASS module r.slope.aspect which calculates several geo-morphological layers, including slope, and you can choose to output either in degrees or percent. It's under in Processing menu, under GRASS -> raster.
But I think you need to first check your "pretty plain" DTM. What is the resolution? Check some cell values and be sure you have a slope at all.

share|improve this answer

edited Jun 6 '14 at 10:44

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

add a comment | 

1

There are a couple of ways to get a slope raster with units in percent. In addition to the 1st answer below by user30184, you can also choose the GRASS module r.slope.aspect which calculates several geo-morphological layers, including slope, and you can choose to output either in degrees or percent. It's under in Processing menu, under GRASS -> raster.
But I think you need to first check your "pretty plain" DTM. What is the resolution? Check some cell values and be sure you have a slope at all.

share|improve this answer

edited Jun 6 '14 at 10:44

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

add a comment | 

There are a couple of ways to get a slope raster with units in percent. In addition to the 1st answer below by user30184, you can also choose the GRASS module r.slope.aspect which calculates several geo-morphological layers, including slope, and you can choose to output either in degrees or percent. It's under in Processing menu, under GRASS -> raster.
But I think you need to first check your "pretty plain" DTM. What is the resolution? Check some cell values and be sure you have a slope at all.

share|improve this answer

edited Jun 6 '14 at 10:44

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

share|improve this answer

edited Jun 6 '14 at 10:44

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

answered Jun 6 '14 at 10:28

MichaMicha

10.8k1321

0

How to change the band from 1 to 4. When i tried to convert DSM to slope only 1 band is coming. But i need 4 bands.

share

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

0

How to change the band from 1 to 4. When i tried to convert DSM to slope only 1 band is coming. But i need 4 bands.

share

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

How to change the band from 1 to 4. When i tried to convert DSM to slope only 1 band is coming. But i need 4 bands.

share

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 8 mins ago

JSimonJSimon

1

New contributor

JSimon is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 8 mins ago

JSimonJSimon

1