I want to be able to draw a circle with 50km radius and then fill the circle with values corresponding to the free space loss formula below:

FSPL(db)=20 log10(d)+20log10(f)+32.44

for example: enter image description here

a circle of 50km around the "west Myara site" and then for every pixel(I guess) I would like to know its corresponding FSPL value and have it colored accordingly.

would I have to convert the polygon to a raster and then use the raster calculator to produce the values of each cell/pixel? if thats the case I would need to find the distance of the pixel/cell from the center point (in this case "west of Myara site" point) and how would I do that ?

  • What f and d represent ? – nickves Jul 19 '16 at 2:08
  • @nickves F=frequency(in MHz)and D=distance(in Km), so distance would be the distance between the cell and the centre point "West Myara..." and Frequency would be any of these 5,6GHz,8GHz,11GHz, and so on(any Microwave tech) – Ashan Lazarus Jul 19 '16 at 3:10
  • How are you going to specify freq? How are they displayed when there are multiples? – BradHards Jul 19 '16 at 3:41
  • I will perform the analysis for each of those frequencies. ultimately I would like to produce such circles for each of our sites(1000's) for each frequency – Ashan Lazarus Jul 19 '16 at 3:53
  • You have a QGIS tag but make no mention of what you have tried to do with that software to answer your question prior to posting it. – PolyGeo Jul 22 '16 at 8:10

The general approach would be:

  1. Rasterize the point "west Myara site"
  2. Generate the buffer (there are plenty of tools for buffering raster, look for "raster distance" or "proximity". QGIS has this algorithm)
  3. Use raster calculator using the formula, f stands for frequency of the wave (a constant value) and d will be buffer layer you calculated previously.
  • Caiohamamura, im trying to convert the vector points to rasters and im not having any luck, all I get is a blank raster file, I have no idea what im doing at this point :( – Ashan Lazarus Jul 19 '16 at 4:50
  • That's tricky, I think there's some bug with gdal_rasterize. Here I need to use gdal_rasterize with -te to specify extents and -tr to specify resolution, then it will be filled with 0. Then you run gdal_rasterize again without setting anything, just the -a field to the same output as previous – caiohamamura Jul 19 '16 at 7:09
  • You may need to set the -a_srs too – caiohamamura Jul 19 '16 at 7:18
  • what values should I use for -te and -tr and in regards to -a the rasterize dialog box wont let me choose the attribute field its locked and greyed out. is -a_srs different from -a followed by attribut column name ? – Ashan Lazarus Jul 19 '16 at 7:28
  • Iv'e found an easier solution, it's a problem of extents when you have only one point the rasterize won't know what will be the extent of the new raster, so add two dummy points surrounding the point of interest and attribute a value of 0 to the field you set in -a. Then rasterize should work. – caiohamamura Jul 19 '16 at 8:06

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