GeofencING USING Geohash

The Outback Navigation Information System (ONIS) Geofence functions use the mathematical Geohash algorithms to determine location boundaries.

Geohash is a public domain geocode system invented in 2008 by Gustavo Niemeyer and constitutes a mathematical model of a 3 dimensional sphere comprising 2 dimensional areas, represented by a single string of characters and digits, named the Geohash.

The precision of the Geohash area is determined by the number of characters in the string and range from 1 to 12 in the mathematical model. 

A unique feature of the Geohash system is that:

• it produces a single character string which is more efficient for location data comparisons

• is hierarchical, as the Geohash mathematical model permits removing characters from the end of the character string to effectively increase the area, e.g. the higher precision Geohash area qd665z is located within the next level down qd665 area.

The ONIS uses the base32 representation for Geohash calculations and refer to them as the ONIS Geofence.

The base32 implementation MUST be used by the owner if they are creating their own Geofence data records.

The ONIS has inbuilt tools (only enabled in the owner mode) to capture, import or export Geofence data.

It is fundamental for the owner to have a design philosophy when implementing a ONIS Geofence system as poorly designed data records can lead to unwanted and unnecessary alarms or messages.

Typically, the owner would limit the use of the Geofence precision to 4, 5, 6 & 7, for both Speed Alerts and Messages, as in most cases the areas of interest will be remote with few (if any) other roads, tracks or areas of interest close by.

Geofence 8 & 9 precision have been included in the ONIS system for the odd occasion that an access or easement track is close to an adjacent main road, and both require substantially different Speed Alerts or Messages.

As the user moves further from the equator, the Geofence squares and rectangles change dimensions due to the difficulty of mapping coordinates of a sphere into two dimensional coordinates.

At the equator, all odd character precision Geofence will be almost square, e.g.

Geofence            Area
precision            width x height
     1                        5,009.4km x 4,992.6km
     3                        156.5km x 156km
     5                        4.9km x 4.9km
     7                       152.9m x 152.4m
     9                        4.8m x 4.8m
    11                     14.9cm x 14.9cm

At the equator, all even character precision Geofence will be rectangular and almost twice the width to height.

Geofence            Area
precision            width x height
    2                        1,252.3km x 624.1km
    4                        39.1km x 19.5km
    6                        1.2km x 609.4m
    8                        38.2m x 19m
   10                      1.2m x 59.5cm
   12                      3.7cm x 1.9cm

This change of Geofence dimensions does not affect the Outback Navigation Information System implementation as the Geofence implementation is used for areas (not distances), and the same algorithm is used to create both the Geofence data records and the current location Geofence.

GPSIS has implemented precision 4 to 9 Geofence in the Outback Navigation Information System (ONIS) as these will be most suited to the ONIS intended usage.

As the Geofence mathematical model permits removing characters from the end of the character string to effectively increase the area, this permits the ONIS owner to overlay different levels of Geofence precision Speed Alerts or Messages, over each other.

The highest precision (most characters) Geofence will always take precedence if more than one record is found.

At highway speeds, the Geofence message system is best suited to precision levels of 7 and below (e.g. larger areas) as the ONIS only performs Geofence calculations and comparisons every 5 seconds (approx.).

EXAMPLE (image above)

The 6 character Geofence qe8fsv is within the 5 character Geofence qe8fs.

The ONIS can now be configured as follows:

• 110 km/h for qe8fs

• 80 km/h for qe8fsv (and others)

When the vehicle enters the larger area of qe8fs, then the ONIS will set the Speed Alert value to 110 km/h.

When the vehicle enters the smaller area of qe8fsv, then the ONIS will set the Speed Alert value to 80 km/h, as higher precision Geofence take precedence.

If the ONIS can not find any matching Geofence data records, then the Speed Alert will return to the last User or Adaptive setting.

The Outback Navigation Information System (ONIS) utilises Geofence Speed Alert & Geofence Message data records which reside in the ONIS Internal memory area.

These data records correspond to a Geofence precision level of 4, 5, 6, 7, 8 or 9 and are either User or System .

User Geofence data records can be configured by using the inbuilt capture tools or using a PC connected to the ONIS, and the import/export tool.

System Geofence data records are secured and only configurable by GPSIS (with information provided by the owner) and installed during the build process.

System Geofence data records will take precedence over user entries if both are found for the same current Geofence location.

We recommend only using use the System Geofence data records if the data WILL NOT change for the life of the ONIS. 

During every 5 second (approx.) navigation scan, the ONIS will perform the Geofence calculations using the current location and search the Geofence tables for matching data records with the following order of priority, from high to low:

• System 9 character precision (highest priority)

• User 9 character precision

• System 8 character precision

• User 8 character precision

                    and so on, until

• System 4 character precision

• User 4 character precision (lowest priority).

If data matching is not found, then there will not be a Geofence Speed Alert or Message displayed.