QZone

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.

For example:

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.

Also see: Creating Polygon Items - Making large Polygons more efficient: QZones

Properties

  • 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.

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.