58
4
G E N E R A L I N F O R M AT I O N
What is 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 units.
What is a 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.
Cobra
®
GPS receivers include 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.
About 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.
4
57
G E N E R A L I N F O R M AT I O N
NOTES
