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We will show in this chapter how to perform an overlay analysis of a
zone over point features. The objective is to find out how many trees are
so close to the utility poles that the branches could be in the way of the
poles. A simple model will be used, which is to assume that branches
extend four metres from the trunk. We will not consider here the height
differences between the branches and the poles. The analysis will be
performed using three steps.
This analysis is different from the previous one is that no user-defined attributes will be used. That is to say, all the analysis will be performed using only system-defined attributes. We could have created databases for the trees and utility poles as well.
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Disconnect from the DB Manager. In this exercise, we will be working with tree features without any user-defined attributes. To prevent the GIS from searching the user-defined database, the DB Manager will be disconnected from the Information Manager.
Use the cursor to select "Info_Manager" in the Project field and press the Delete key on the keyboard to delete it. Also turn off Auto Retrieve and Auto Send to disconnect the two databases.
NOTE : To connect with the Info_Manager database again, simply return the Selection Settings panel to its previous state. Generate buffer zones around all trees.
Retrieve utility poles within the zone. 1. Display only utility poles (feature code is UTILITY_POLE).
(288824, 740226), 1:300 How many trees are too close to the utility poles ?
Overlay of zone over points. Out of overlays of zone over zone, over line, and over point, overlay of zone over point is the simplest. The zone is stored in raster format, and the pixels belonging to the zone are known. To find out if a point feature is overlapping a zone, we need only to check if a pixel in the zone already exists at that location. If the zone is a vector polygon, many more computations would have to be performed to determine whether a point is inside it or not. For an explanation, please consult the technical note in the chapter on Use Topology to Identify Polygons by Pointing. |
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