Bounding Box

The bounding box is the smallest rectangle that will completely enclose a feature.  Two points (x-y coordinates) will determine it, or alternatively the MaxX, MaxY, MinX, and MinY. A Shapefile stores the bounding box for every record, and for the entire database.

Four line segments, each of which would have the same bounding box.  The corners of the box would be the SW and NE corners.

The line segments have a direction, since they might have attributes based on their topology, or right and left sides (such as the addresses in TIGER roads).   The direction goes from the first point in the coordinate list, toward the last point.

A more complex line with its bounding box in black, and a screen window in green.

Without the bounding box, a program would have to consider all coordinates in the line (potentially thousands) to know if the line or any part of it is on screen. 

With the bounding box, programs can do a quick check and reject many lines as having no possibility of being on screen, such as the lower left case in the diagram to the left..

 

It is possible that the screen will lie partly or wholly within the bounding box for a line, and yet the line will be completely off the screen.  In this case the GIS program will have to project the line, and then determine point by point that it does not plot on the screen.

ESRI shapefiles store the bounding box in the SHP file for each line and area record.  MICRODEM also adds this information to the DBF file.


Last revision 1/8/2018