What Is a Bounding Box in Mapping?

A bounding box is the smallest rectangle, aligned with the coordinate axes, that completely contains a place, a shape or a group of points. It is described by four numbers: the southernmost and northernmost latitudes, and the westernmost and easternmost longitudes. It is a simple, fast way to say roughly where something is.

The four values

A bounding box is often written as:

west, south, east, north

or, depending on the tool, south, west, north, east. The order varies, which can cause the same type of confusion described in latitude first or longitude first. Always check the documentation. In this form, a point is inside the box if its latitude is between south and north and its longitude is between west and east.

Why they are useful

• Quick filtering: testing whether a point is in a rectangle is much cheaper than testing against a complex outline, so boxes are used to reject most candidates first.
• Map views: the visible part of a map is a bounding box. Apps use it to ask a server for only the places in view.
• Search areas: a search can be restricted to a region by giving a box.
• Fitting data: a map can be zoomed to show all selected points by computing the box that contains them.
• Indexing: spatial databases store a box for each feature to speed up searches.

An example

A search for cafes in a city centre might send a box such as 51.50, -0.14 to 51.52, -0.10. The server returns only cafes with latitude and longitude inside those limits. The box covers about 2.2 km north to south, because 0.02 degrees of latitude is roughly 2.2 km; see how long a degree is.

Limitations

• It is crude. An L-shaped or diagonal region has a large box with empty corners. Points in those corners are inside the box but outside the real shape.
• The 180 degree meridian. A region that crosses the line where longitude jumps from 180 to -180 cannot be described by a single simple box in the usual way, so software must handle this case.
• Not equal in area. A box defined in degrees covers less ground near the poles.

Boxes and grid codes

A grid cell is effectively a very small bounding box. A UPCS code corresponds to a cell with bounds (north, south, east and west) and a centre, and the embeddable map returns the selected cell in this form. A geofence may also start as a bounding box before more exact shapes are used.