With Wildland Fire season coming upon us it is a good opportunity to review our mapping standards. For our purposes most of us do not need detailed geospatial training but we can benefit from an understanding of two topics: Grid Systems and Reference Datum. I know this is starting to sound like your 7th grade math class, so bear with me.
A Grid System allows the location of a point on a map to be described in a way that is meaningful and universally understood. The common Grid Systems used in the United States are:
- Geographic - uses degrees of latitude and longitude.
- Public Land Survey - used in Colonial America for surveying. You might know it as Township/Range.
- UTM - Preserves shape, and allows for precise measurements in meters.
- State Plane - Developed for local surveying, with minimal distortion.
For our purposes we use a Grid System to tell someone where we want them to land to pick up a patient or drop a load of water or retardant. When communicating location use the Geographic Grid and provide LATITUDES and LONGITUDES in Degrees, Minutes and Seconds (DMS). The following examples show how different GPS units positioned at Station 1 might display coordinates in DMS:
· 37° 8' 24.312"N 108° 71' 21.761"W
· N 37° 8' 24.312" W 108° 71' 21.761"
· 37° 8' 24.312" -108° 71' 21.761"
· 37 8 24.312 -108 71 21.761 (no units)
A Reference Datum is defined as a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference datum is sea level. On other planets, such as the Moon or Mars, the datum is the average radius of the planet. The term "reference datum" was used rather than ‘above (or below) the earth’s surface’ or ‘above (or below) sea level’.
As with Grid Systems, there are numerous Reference Datum. Most of the Topographical Maps still in circulation use the NAD-27 Datum. (North American Datum, 1927). However, the WGS 84 datum is the most widely used datum in the world and is what we use in our GPS units.
To give you some background, WGS 84 is based on the combination of Doppler satellite images and 1980 Geodetic Reference System (GRS 80) to create a new, more accurate world geodetic system. In terms of reference, WGS 84 uses what is called the "zero meridian" but because of the new measurements, it shifted 100 meters to the east from the previously used Prime Meridian in Greenwich. WGS 84 replaced WGS 72 and is valid until 2010.
In summary, our GPS units located in our apparatus are programmed to use Degrees, Minutes, and Seconds based on and WGS 84 datum. If you have a personal GPS unit used during incidents, please make sure it is programmed the same way.
By complying with these standards we know we are providing proper information to Durango Interagency Dispatch, Fire Dispatch, Care Flight, and Air Care. We also know the data we collect during an incident is consistent for mapping analysis purposes.