A QZone item is a 2D region represented by a hierarchical grid of squares that fit together to cover a region.
Note: The Q in QZone stands for "Quadtree".
This is a very compact and fast way to store and analyse complex regions. Each square can be divided into four quadrants and each quadrant sub-divided into quadrants repeatedly until they are uniform or the basic resolution of the data is reached.
Because the squares that make up the QZone are regularly sized, it is an extremely efficient way for the computer to model a 2D region.
Detecting whether a point is inside a large QZone is hundreds of times faster than detecting whether the point is inside an equivalent Polygon item.
A drawback of using QZone items is they have jagged edges (especially when zooming in). When you create a QZone you can specify the size of the smallest square. By picking a small value you can stop the QZone appearing too jagged.
- Area: _area#
The area covered by the item in square metres.
- Clockwise: _bClockwise&
Is the item geometry defined in a clockwise direction?
- Data size: _sizeData&
The number of bytes used to store the QZone squares.
- Normalised: _normalised&
Is the QZone normalised? All QZone items are created normalised, but if you move them they become de-normalised.
Before QZone items can be combined in Boolean operations with each other they must be normalised. When they are normalised their squares overlap. If SIS needs a QZone to be normalised for a Boolean operation then it will normalise it on-the-fly. But you can speed up Boolean operations by normalising QZone items after moving them.
- Resolution: _resolution#
The resolution of a Polygon, QZone, or Bitmap item.
- For a Polygon item, the value is the resolution of the invisible cached QZone item. If there is no cached QZone item then the Polygon item's resolution is reported as zero.
A QZone item is made up of progressively smaller squares. Its resolution is the size of the smallest QZone square.
The resolution of a Bitmap item is the size of each pixel.
- Splinter factor: _splinter#
The splinter factor of the Polygon. A circle has a splinter factor of 1. A very long and thin area could have a splinter factor of 100. Polygon items with large splinter factors are often produced by overlapping lines which are almost but not quite the same.
TIP: When defining a cell size for a QZone, you need to make a compromise between polygon accuracy and the speed of the query. Try and match the cell size to the capture scale of the data to obtain as accurate a match as possible.