Re: [Qgis-user] Calculating area and length from DTM

[Lene Fischer]

Hi Again,
I have found tree solutions for my problems- Can anyone tell me if they are 
correct.

I clip the DTM with the polygon and the use the SAGA Real surface area. It 
gives me a new raster where the pixelvalues has changed from Z to a calculated 
area. Finally I use the Zonal statistic and add the SUM value to the polygon.

The second is a Grass function r.surf.area It gives me an area in the log.

The functions has a slightly different result, but it´s only 3/1000 :-)
--

For calculating the fence I had to be a bit more old school. Distance= 
SQRT((x2-x1)^2 + (y2-y1)^2 + (z2-z1)^2) I took the polygon and converted it to 
points with a ordered pointnumber. 
Used the SAGA function Add Grid Values to Point It takes the Z Value from the 
DTM and add an attribute to a new pointlayer. 
The pointlayer is saved as CSV. It gives me a table with x,y and z.
And in Spreadsheet I can easily calculate the length af each vertice and make a 
sum. :-)


Regards
Lene Fischer
--
Sendt: 31. januar 2016 17:52
Til: qgis-user
Emne: [Qgis-user] Calculating area and length from DTM

Hi,
Today I have been out collecting data with my GPS in an area with huge 
differences in heights.
My length and area will be from the "flat" area and not accurate/correct.

If a contractor should order a fence - it would be too short :-O

How do I calculate correct when I have the polygon and a terrain model.

Regards
Lene Fischer
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Re: [Qgis-user] Calculating area and length from DTM

[Paolo Cavallini]

Thanks Lene. Why not creating a model and adding it to shared repo?
All the best.

Il 1 febbraio 2016 09:33:47 CET, Lene Fischer  ha scritto:
>Hi Again,
>I have found tree solutions for my problems- Can anyone tell me if they
>are correct.
>
>I clip the DTM with the polygon and the use the SAGA Real surface area.
>It gives me a new raster where the pixelvalues has changed from Z to a
>calculated area. Finally I use the Zonal statistic and add the SUM
>value to the polygon.
>
>The second is a Grass function r.surf.area It gives me an area in the
>log.
>
>The functions has a slightly different result, but it´s only 3/1000 :-)
>--
>
>For calculating the fence I had to be a bit more old school. Distance=
>SQRT((x2-x1)^2 + (y2-y1)^2 + (z2-z1)^2) I took the polygon and
>converted it to points with a ordered pointnumber. 
>Used the SAGA function Add Grid Values to Point It takes the Z Value
>from the DTM and add an attribute to a new pointlayer. 
>The pointlayer is saved as CSV. It gives me a table with x,y and z.
>And in Spreadsheet I can easily calculate the length af each vertice
>and make a sum. :-)
>
>
>Regards
>Lene Fischer
>--
>Sendt: 31. januar 2016 17:52
>Til: qgis-user
>Emne: [Qgis-user] Calculating area and length from DTM
>
>Hi,
>Today I have been out collecting data with my GPS in an area with huge
>differences in heights.
>My length and area will be from the "flat" area and not
>accurate/correct.
>
>If a contractor should order a fence - it would be too short :-O
>
>How do I calculate correct when I have the polygon and a terrain model.
>
>Regards
>Lene Fischer
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>List info: http://lists.osgeo.org/mailman/listinfo/qgis-user
>Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-user
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Re: [Qgis-user] Calculating Area. differences when transformation on the fly on / off

[Nicolas Cadieux]

Hi, 
To make sure you don't get mistakes, it is best to have the project CRS and 
layer CRS the same. 
As your project CRS is in meters, the results are in meter on that reprojected 
file. Some tool work exclusively with the layer CRS while some tool can work 
with reprojected files. 
In your case, when on the fly is off, the tools uses the layer unit. When 
on the fly is on, the layer is reprojected and the tool used the project CRS. 
Nicolas Cadieux M.Sc. 
Les Entreprises Archotec inc. 
8548, rue Saint-Denis Montral H2P 2H2 
Tlphone:514.381.5112 Fax: 514.381.4995 
www.archeotec.ca 
On Jan 26, 2016 11:51, kmithoefer [via OSGeo.org] 
ml-node+s1560n5247360...@n6.nabble.com wrote: 


  


  
  
Dear all, 
  
  I calculate area values for counties in Alaska. The layer and the
  project is in EPSG:2964 - NAD27 / Alaska Albers. However, If I
  switch transformation on the fly on I get values in
  square-meters. If I switch it off I get squarefoot, which is
  correct because the unit for EPSG:2964
  - NAD27 / Alaska is Foot_US. 
  So where are the square meters based on and what drives the
  calculation, the layer CRS or the project CRS. 
  
  Thank you 
  Cheers 

  Klaus 
  
  -




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Re: [Qgis-user] Calculating area from a slope

[G. Allegri]

A simplified approach could be calculating the projected area:
http://en.wikipedia.org/wiki/Projected_area

giovanni


2014-06-23 17:04 GMT+02:00 Lene Fischer l...@ign.ku.dk:

 Hi
 I just got a question from a student - and my own brain is in middel of
 exams - so I hope some of you can help:

 We have a DSM over roofs in our area. We have made slope calculations. How
 can we calculate the area of the roofs.
 Regards
 Lene Fischer
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Re: [Qgis-user] calculating area

[Nik Go]

Thank you all for the tips. I've actually managed to do the same when
I changed the CRS to the appropriate UTM.


On 11/21/09, John C. Tull jct...@gmail.com wrote:
 Hi Nik,

 On Nov 20, 2009, at 12:04 AM, Nik Go wrote:


 Hello. I'm trying to calculate area of my shapefiles but I'm not really
 sure if I'm doing it right.  I expect results to be in meters (sq meters
 or sq. kilometers) but read out is in sq. degrees.  I'm told that I have
 shapefiles in geographic coordinate system WGS 84 but I don't know to
 convert this to a projected coordinate system from qgis.

 So, how should I go about doing
 converting from geographic coordinated to projected coordinates so I could
 determine/calculate area
 determine distance between points
 convert sq. degrees to sq. meters?

 You can change your project CRS from the default geographic coordinate
 system of WGS84 to a projected coordinate system based on meters, such as a
 UTM zone. Turn on the Enable 'on the fly' CRS transformation, and you will
 be set for vector data. If you have raster data, Qgis does not handle on the
 fly transforms on them. For those, I recommend looking at gdalwarp, part of
 the gdal tools. In particular, look at the vrt output format so you do not
 create numerous large file. A vrt invokes gdal to, effectively, perform an
 'on the fly' reprojection of the original raster data.

 Regards,
 John
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Re: [Qgis-user] calculating area

[Micha Silver]

Nik Go wrote:



Hello. I'm trying to calculate area of my shapefiles but I'm not 
really sure if I'm doing it right.  I expect results to be in meters 
(sq meters or sq. kilometers) but read out is in sq. degrees.  I'm 
told that I have shapefiles in geographic coordinate system WGS 84 but 
I don't know to convert this to a projected coordinate system from qgis.
If your data is in a geographic CRS, then you probably don't want to use 
area calculations. It's best to convert the data to a projected CRS, 
then do the area, length calculations.

Using the fTools plugin:
-  under the Tools-Data Management menu is the tool to Define the 
projection. Here you can set the projection of your original data to 
WGS84 Lat Lon (EPSG:4326). This is NOT changing your data, only 
declaring its original projection information.
- Now, again under Data Management, choose Export to a new projection. 
This alllow you to create NEW shapefiles, translated to some projected 
CRS of your choice.
- And next, under the Geometry tools choose Export/Add geometry 
columns. THis will create a third shapefile, this time with added 
attribute columns. If the shapefile was of polygon (area) features then 
this tools calculates area and perimeter columns for each feature.


Distance between point is another discussion. ;-)

HTH,
Micha



So, how should I go about doing

* converting from geographic coordinated to projected
  coordinates so I could
  o determine/calculate area
  o determine distance between point s
* convert sq. degrees to sq. meters?


Thanks



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