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I want to study 6 areas in the field and in order to do so I want to divide each one of them into smaller areas/parts.

What I would like to do is find an average square or rectangle area surface (more particularly I need both figures) and then add 4 buffers of 50m each (meaning a total of 200m buffer zone), in each of my study areas. All surfaces could be made using fishnet of 50mx50m too. I guess I can add data in each polygon / fishnet cell, by editing the attribute table in each polygon after it is created. I am not a GIS expert, so indicate the steps in order for me to reproduce each procedure (creating shapefiles, editing vector, making the buffer etc).

I have georeferenced google earth images of my areas using QGIS 2.4. I have done a little search in this site for possible answers, but quite many of these solutions do not work in the QGIS I use; either errors occurred or do not suit my case.


I am adding some pictures to give a visualisation example of what I would like to do, made by using a presentation (powerpoint) program. I am giving 2 examples for each case, rectangle or square. With blue is my main area and with orange the buffer zone(s). In case of fishnet, every eye should be 50mx50m. In the other case (if fishnet is not possible) there could be polygons with the same snapping point and a 50m distance in between each polygon's side with its closest one. Both surfaces, rectangle or square, enlosed by blue line should cover the same area in sqm.

below you can see the examples:

  1. rectangle with fishnet
  2. rectangle with polygons (if fishnet option is not applicable)
  3. square with fishnet
  4. square with polygons (if fishnet option is not applicable)

rectangle with fishnet rectangle with polygons (if fishnet option is not applicable) square with fishnet square with polygons (if fishnet option is not applicable)

  • What do you mean find the 'average area surface'? – camdenl Oct 6 '14 at 16:41
  • I would like to have the same area surface for all of my study areas (some are streets and some square-like). So depending on how my areas look like, I need them to have let's say 6000sqm, either in square or rectangle shape. And then create the buffer zones based on them. I do not know if it is easier to make the main polygon for each area based on a single point or create a polygon and adjust it to my area. Thank you. – Thanasis Oct 6 '14 at 16:55
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    I'm still having a hard time understanding your question, do you want to create multiple overlapping buffers and then extract values from a raster? – camdenl Oct 6 '14 at 18:05
  • Please consider adding a sketch of what you want to do. It's difficult to follow the question. – underdark Oct 6 '14 at 18:26
  • I'm still uncertain what rules should be applied to "find an average square or rectangle area surface"? For the fishnet solutions, wouldn't it be enough to generate a grid and move it to your desired location manually? Or does it need to be some kind of automatic process? – underdark Oct 7 '14 at 12:01
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You can create square and rectangular grids using the Vector Grid tool under Vector > Research Tools > Vector Grid.

To get the required coordinates, I suggest using the Coordinate Capture tool and then input the Xmin, Xmax, Ymin, Ymax from these captured points. Note that the units will be in the coordinate system currently used, so you might want to reproject your raster into a coordinate system with Meters. To create a rectangular grid, simply unlock the 1:1 ratio on the tool menu. You can create the grids as both polygons and polylines

Here's an example of the Vector Grid parameters:

vector grid parameters

and the output:

output

  • Thank you for your answer. Can I orient this grid and move it? Can these grid cells be treated separately? (e.g. different colour line, transparency of filled colour). Can I specify how many cells will the grid have? Can I add data in each cell (attribute table)? If yes, how? I need to create the polygon(s) / grid based on the middle point of my areas, rather than the four min and max points. – Thanasis Oct 7 '14 at 16:15
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    Perhaps using the tool would answer some of these questions? – camdenl Oct 7 '14 at 16:18
  • This method does not help me do (and also analyse) what I would like to do. My georeferenced images are in Greek Grid (2100) projection system, but when I try to use this function (Vector Grid) I do not know what the parameters should be and also I do not know what Xmin,Ymin,Xmax,Ymax should reffer to. I also suppose that the output grid should be selected as polygons, so that I can put info in it. – Thanasis Oct 8 '14 at 10:56
  • Dear @Thanasis, what do you think Xmin,Ymin,Xmax,Ymax could refer to? If you need somebody to do the whole analysis for you, you might have to hire a GIS analyst. – underdark Oct 8 '14 at 14:00
  • Firstly, I would like to apologise to all for my ignorance on GIS matters. It is not my field and I wrote in my first message that I am not an expert and I need thourough guidance. Thank you all for your answers and time. @underdark, bad choice of words. I apologise. I meant where these points should be placed. Since I want to create a grid with certain numbers of cells of specified size based on a line or point, I cannot know to where the grid can extend exactly, so that I put the correct values. – Thanasis Oct 8 '14 at 22:25

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