About GPS Data
Note: This is an advanced topic and is meant for those interested in the subject. The average Aptora software user will not need to know this information.
Aptora Mobile II and Aptora GEO Time Tracker return GPS coordinates known as Decimal Degrees. Our software may also collect GPS information from third-party software such as Verizon’s GPS tracking systems.
What Are Decimal Degrees?
Decimal degrees (DD) express latitude and longitude geographic coordinates as decimal fractions and are used in many geographic information systems (GIS), web mapping applications such as OpenStreetMap, and GPS devices. Decimal degrees are an alternative to using degrees, minutes, and seconds (DMS). As with latitude and longitude, the values are bounded by ±90° and ±180° respectively.
Positive latitudes are north of the equator, negative latitudes are south of the equator. Positive longitudes are east of Prime meridian, negative longitudes are west of the Prime Meridian. Latitude and longitude are usually expressed in that sequence, latitude before longitude.
Valid Range of Numbers
The valid range of latitude in degrees is -90 and +90 for the southern and northern hemispheres respectively. Longitude is in the range -180 and +180 specifying coordinates west and east of the Prime Meridian, respectively.
The Prime Meridian has a longitude of 0° that goes through Greenwich, England. The International Date Line (IDL) roughly follows the 180° longitude. A longitude with a positive value falls in the eastern hemisphere and the negative value falls in the western hemisphere.
Decimal Degrees Precision
Six (6) decimal places precision in coordinates using decimal degrees notation is at a 10 cm (or 0.1 meters) resolution. Each .000001 difference in coordinate decimal degree is approximately 10 cm in length. For example, the imagery of Google Earth and Google Maps is typically at the 1-meter resolution, and some places have a higher resolution of 1 inch per pixel. One meter resolution can be represented using 5 decimal places so more than 6 decimal places are extraneous for that resolution. The distance between longitudes at the equator is the same as latitude, but the distance between longitudes reaches zero at the poles as the lines of meridian converge at that point.
If Latitude value is reported as -6.3572375290155 or -63.572375290155 then you could round-off and store up to 6 decimal places for 10 cm (or 0.1 meters) precision.
For millimeter (mm) precision then represent lat/long with 8 decimal places in decimal degrees format. Since most applications don’t need that level of precision 6 decimal places are sufficient for most cases.
In the other direction, whole decimal degrees represent a distance of ~111 km (or 60 nautical miles) and a 0.1 decimal degree difference represents a ~11 sq. Km area.