I want to map a tree survey. I am thinking that I will plot positional points of the trees and then add other data like tree type etc. to the attribute table later.
Is there a way I can have an attribute that describes a radius or diameter, whichever is easier, of a circle round the point?
In fact I need two circles, one to describe the canopy radius/diameter and another to do the same for the root protection area.
I am using QGIS 1.8.0
Best Answer
This can be easily achieved using Advanced - Size scale field (instead of calculating buffers which have to be recalculated every time a value changes).
The idea is to have a simple point symbol of size 1 map unit. (Map units are defined by the project CRS.) If you measure the diameter of the tree top in meters, you can simply use this diameter field as the size scale field.
Size scaling can only be applied to the symbol as a whole. If you want to visualize tree top and root protection area at once, you'll have to add the layer twice and change the size scale field to the other field.
Here is an example of setting I used with the Viennese tree cadastre: