# Visualising triangulation in QGIS

I teach a non-geographical technical course, and the topic will be triangulation (..of suspects via mobile phone signals while using an archived well-known case). Since the system in questioned is GSM, we/they used the TA value (distance from tower quantized from [0:63], where each step is ~550 meters). So if TA is (eg.) 10, the distance from the tower is (in an ideal case) from "10*550m" to "10*550m+549.99m".

I have coordinates of three towers in lat,lon (WGS84), I have the TA values, and I would like to draw three donut shapes (inner radius=ta*550m, outer (ta+1)*550m) which would ideally intersect in one relatively small area. I also have a raster map (satellite images). I have already verified the raster layer and the basestation points are visualised OK, but I have no idea how to draw the 'donuts' (except photoshop and eyeballing it).

What would be the simplest way to visualise that in QGIS?

• Welcome to GIS StackExchange! I have few questions: 1) Do you have a TA value for each tower? If yes, are they stored somewhere or you haven't a digital source? 2) In which UTM zone are your towers? Their coordinates are stored somewhere or you haven't a digital source? 3) A possible solution could be proposed with a Python script? Thanks.
– mgri
Dec 14 '16 at 8:33

You could use the Multi Ring Buffer plugin to get your 'donuts'.

As it's in meters, you should use a UTM CRS, rather than one whose units are in degrees. I'm in the UK so I used OSGB (27700) for this example. You WILL need to project your lat/lon coordinates into a UTM projection - which one depends on where your masts are in the world.

Set up each mast on its own layer - one point per layer. Run the plugin on each mast layer as follows, using settings like this:-

You should now have three sets of concentric circles/rings, like so. Each ring is 550m wide.

Now, on each mast's concentric circle layer, count out the rings from the middle - a TA value of 0 is the innermost circle, 1 is the next band out, 2 is the one after that, and so on.

Use the select tool to select that ring. Save the circle as a new layer, making sure you select "save only selected features" (this is very important) and also make sure you save the layer using the UTM projection.

Once you have that for all three layers, you should see something like this. I chose a TA value of 4 for all 3 masts, but you could use a different TA for each mast if you wanted to.

To get the intersection of all 3,

• select the ring layer from mast 1 and use intersect with the ring layer from mast 2
• select the output from the last step, and use intersect with the ring layer from mast 3.

You should now have the shape of the zone where all 3 overlap...