A P P E N D I X
WAAS, Map Datum, North Reference
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WAAS
Wide Area Augmentation System (WAAS) is a GPS-based navigation
system that provides precision above and beyond what GPS can do
on its own. WAAS was designed to improve the accuracy and ensure
the integrity of information coming from GPS satellites.
WAAS is a network of 25 ground reference stations that cover the
entire U.S. plus parts of Canada and Mexico. Implemented by the
Federal Aviation Administration (FAA) for aviation users, these reference
stations are located at precisely surveyed spots and compare GPS
distance measurements to known values. When the WAAS signal is
available, WAAS-capable receivers typically have a much higher
accuracy than non-WAAS receivers.
Map Datum
Maps and charts are essentially grids created from a starting
reference point called a “datum.” Many maps being used today
were originally created decades ago. Over the years, technology has
allowed us to improve our surveying skills and create more accurate
maps. However, there is still a need to adapt GPS receivers to use
with older maps.
Most navigational charts and maps will have the datum listed. This
GPS receiver includes up to 100 map datums that allow you to
switch to a setting that matches your map. The most common US
map datums are World Geodetic System 1984 (WGS 84) North
American Datum 1983 (NAD 83) and North American Datum 1927
(NAD 27)
NOTE:
Using a map datum that does not match the chart you are
using can result in significant differences in position information.
North Reference
The vertical grid lines on any map can be aligned in one of three
different ways.
“True North” means that the vertical lines are aligned with the
geographic north pole. This is the most common method of
orienting maps.
“Magnetic North” means that the vertical lines are aligned in the
direction indicated as north by a compass, which can differ from True
North to varying amounts, depending on where you are on the Earth
(this difference is called “inclination” and “declination”.)
“Grid North” refers to the direction in which the straight lines on a
map are actually aligned, which may be slightly different from True
North, because the rounded surface of the Earth can not be
accurately depicted on the flat surface of a map.
