FAQ

1. What is viewshed?
2. What is Fresnel zone?
3. What is SRTM?
4. How to get free SRTM DEM?
5. How to import SRTM DEM into GRASS GIS?
6. Why mouse hover doesn't work in src_gui_grass/main.cpp?

1. What is viewshed?
A viewshed is the geographical area that is visible from a location. It includes all surrounding points that are in line-of-sight with that location and excludes points that are beyond the horizon or obstructed by terrain and other features (e.g., buildings, trees). Conversely, it can also refer to area from which an object can be seen. A viewshed is not necessarily "visible" to humans; the same concept is used in radio communications to indicate where a specific combination of transmitter, antenna, and terrain allow reception of signal.
See: wiki viewshed

2. What is Fresnel zone?
In optics and radio communications (indeed, in any situation involving the radiation of waves, which includes electrodynamics, acoustics, gravitational radiation and seismology), a Fresnel zone, named for physicist Augustin-Jean Fresnel, is one of a (theoretically infinite) number of concentric ellipsoids which define volumes in the radiation pattern of a (usually) circular aperture. Fresnel zones result from diffraction by the circular aperture. The cross section of the first (innermost) Fresnel zone is circular. Subsequent Fresnel zones are annular (doughnut-shaped) in cross section, and concentric with the first. See: wiki Fresnel zone

3. What is SRTM?
Freely available SRTM data are freely available on the website USGS Home. For the purpose of testing we have used the latest NASA SRTM3 SRTMGL1. The resolution of the data is one arc second. Previous versions of SRTM (1.0 and 2.1) had undefined areas of the data caused by some characteristics of radar methodology. The problem arises on a stepped land or in areas with low reflection, such as deserts. In SRTM3 are undefined areas corrected by other data sources.

4. How to get free SRTM DEM?
Free SRTM DEM is available at the USGS Home. First, you must log in. If you don't have an account, create it, its' for free.
Procedure for get data is following:
Zoom to required area. Click button Use Map. You'll get all four locations of the corner of your selected area. If you zoom out, you can clearly see selected area. Then click button Data Sets. In new dialog Data Set Search put SRTM (without Enter) and scroll down and select (All Matched Results). Then click button Results. A data Set dialog appear. If available for your area, try to choose a NASA SRTM3 SRTMGL1 data set. At each data set, click on Add to Bulk Download icon (small brown icon with even smaller arrow on lower left corner). Click button View Item Basket and then Proceed To Checkout. At the end click on button Submit Order. Your order is now waiting for Bulk Download.
You can install Bulk Download application from bda. Run application from command prompt: bda. First you must login and then select the order. Select destination (at the top just bellow pull down menu) and then click button Begin Download. It will take some time to complete.

5. How to import SRTM DEM into GRASS GIS?
SRTM data are distributed in *.HGT files size 1201 x 1201 grid cells in resolution 3 arc seconds and *.HGT files size 3601 x 3601 grid cells in resolution of one arc second. For larger areas it is necessary to save the adjacent areas (ziped *.HGT files) in a common directory and run the script:
for i in *.hgt.zip ; do unzip $i ; done
gdalwarp *.hgt my_srtm.tif
gdalwarp -of GTiff -co "INTERLEAVE=PIXEL" -s_srs "+proj=latlong +datum=WGS84" -t_srs "+proj=utm +zone=33 +datum=WGS84" my_srtm.tif my_srtm_mercator.tif
The first command unzip all files. The second command all the *.HGT files glue together and make result in geo-tif format. The third command makes a new geo-tif file that uses the UTM projection (be sure to choose right zone: Spatial Reference). Import geo-tif file into the GRASS GIS environment can be done from GRASS GIS with command:
r.in.gdal input=my_srtm_mercator.tif output=my_srtm
Details are in Installation guide.

6. Why mouse hover doesn't work in src_gui_grass/main.cpp?
A solution is pretty odd, but it works. Copy main.cpp from project/src_gui_grass to project/src directory. Run nsight and in Project Explorer press F5 to refresh. Right click on main.cpp and choose Index/Freshen All Files. Then open main.cpp in nsight editor. Mouse hover should work. Then close main.cpp and close nsight. Delete main.cpp in the project/src directory. Again run nsight and in editor open main.cpp in the project/src_gui_grass directory. Mouse hover should now work.