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Colorado Hiking
GPS your personal navigation system
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Show your current position (and
other preloaded locations) on a handheld unit
Guide you to the trailhead from
which you begin your hike.
Guide you on the trail which you
want to hike by showing your position, and positions defining the course of the
trail.
Show
yo
Show you the track that you hiked
in on - which you can then follow back out to the trailhead.
GPS units have other features that can furnish a lot of additional information, but the things listed above are the real assets to hikers and other outdoors people.
GPS stands
for "Global Positioning System". It was developed and is still
maintained by the U.S. military. The first satellites went up in 1978. The first
most people heard of it was in connection with U.S. military operations in the
Gulf War of 1990/91. Since that time, GPS has become widely used in industry,
and increasingly used for recreation. GPS and navigation are somewhat technical topics. In the
following discussion, I'll keep things as plain and relevant to outdoor
recreation as possible.
Basically,
the Global Positioning System uses satellites orbiting over 12,000 miles above
the earth to determine the position of the GPS handheld unit on the ground.
It does this by using travel times to the GPS unit of signals being
emitted from some of the 24 Navstar satellites (28 total in orbit with four
being spares). At a given time at a specific location, the unit will not be able
to receive signals from (or "see") all of these satellites. With
consumer grade GPS units, at least three are needed to be seen to calculate your
"XY" or map coordinate position. At least four are needed to be able
to calculate altitude.
The
accuracy of a GPS's determination of location will vary depending on a number of
factors, but it is usually within 30 feet or better for the "X, Y" or
map position. The altitude
determination ("Z") is
usually less accurate - geometrically it's just a tougher calculation to get precise. For most applications it's
the map position which is most important, and for hiking 30 feet of accuracy is
just fine.
A number of
factors affect the ability of a GPS unit to determine its position. The
locations of the Navstar satellites are obviously important.
At the particular time you are at a locality, are they clustered
overhead, mostly down near the horizon, or are they evenly arrayed from horizon
to overhead? A wide geometric array provides the best situation for accurate
calculation of positions. Are they
clustered in one direction or spread around the compass? Being spread around
leads to better accuracy. Are there any physical elements that preclude the GPS
unit from receiving a satellite's signals (or "seeing" it)? Mountains, canyon walls, buildings and dense forest can limit
the number of low in the sky satellites that are visible and therefore usable by
the GPS unit. Likewise, signal reception from satellites located overhead can be
limited by dense forest cover. The degree to which this occurs depends on the
lateral density and thickness of the tree cover, leaf types, and moisture on the
leaves (water, snow, ice).
When a GPS unit can get signals from enough satellites to calculate a
positional fix, there are still a number of technical factors that affect the
precision of that calculated position. In
order of declining importance these are: atmospheric
effects, orbit estimation errors, satellite clock drift, multipath interference
and receiver noise.
Some
methods have been developed to partially compensate for these errors. However,
we won't go into it here except to mention that the WAAS feature which is found
on most GPS units is one method designed to deal with this in North America. The
added precision given by WAAS probably cuts the average error down to about 10
or 15 feet - when you can get it to work. Using
WAAS introduces another set of calculations from some satellites in orbit above
the earth's equator – possibly slowing position calculations and using extra
power. In Colorado you need an unobstructed lower angle view to the south, which
you often won’t get. So, these WAAS satellites can't always be
"seen" by the unit and using it is usually not necessary or beneficial
for recreational activities. Most people wind up turning the WAAS feature off.
GPS location fixes or map positions.
The fundamental
thing a GPS unit produces is a locational fix, usually along with an
altitude.
There are a
two things that you need to be aware of concerning a locational fix or map
position. The first is the
"datum" used. The second is the "coordinate system".
Every position is associated with a datum and a coordinate system.
When you set up a GPS for use, you'll
be asked to choose the datum and coordinate system that you want to use.
The earth
is not a perfect sphere and its shape is not perfectly uniform. This creates the
necessity for developing mathematical models (datums) that closely approximate the shape
of the earth over a given region, to facilitate accurate mapping in that region.
Every few decades a new improved datum is devised and adopted. If you look at a
list of datums in use across the world, you'll notice that some of them are
named after the countries and regions in which they are used.
Currently
in the United States, there are three datums that you should be aware of in the
course of using a GPS.
NAD 27
(North American Datum 1927).
NAD 83
(North American Datum 1983).
WGS 84 (World Geodetic System
1984).
NAD 27 is an old datum that was
used in creating many topographic maps that are still in use today.
NAD 83 is the datum that replaced it and that has been used to generate
more recent maps. WGS 84 is the default standard used in the U.S. for GPS
operations.
NAD 83 and
the WGS 84 datums are so similar to each other that the positional difference is
usually negligible. In other words,
if you took a GPS location with the unit set up to use WGS 84, and then plotted
it on an NAD 83 datum map (we'll assume they both have the same coordinate
system) without converting it to NAD 83, the error would be very small. If you
made the same mistake with an NAD 27 map, the error probably would be large
enough to be noticeable.
Good maps
will have a note showing what datum they were generated with. USGS topographic
maps are either NAD 27 or 83. The
Delorme "Atlas & Gazetteer" maps are WGS 84.
So, what
does this mean to you, if you are a GPS user?
Usually nothing other than knowing that WGS 84, NAD 83 or NAD 27 are OK
datum selections if you are using a GPS unit in Colorado (or the rest of the
US). For most purposes, WGS 84 is
the best to use. However, if you ever want to exchange location points between
your GPS and a map, it becomes important to maintain consistency or to make
conversions. Whether it's a paper map or a map graphic program in your computer,
you will want to be aware of the datums, and perhaps change your GPS
datum setting to be consistent with the map, or to make any necessary
conversions from one datum to another before plotting. Likewise, when positions
are shared among GPS users, the datum should be included.
Coordinate
systems are used to describe a location on the surface of the earth. They take
the form of an "X, Y" in degrees or units of distance (depending on
the coordinate system) that can be used to crosshair a position onto a map.
There are
many different systems that are designed to be optimal in various parts of the
world and to minimize the types of distortions involved in representing
locations from a spheroid (earth), on a flat map.
The only
two coordinate systems that somebody using a GPS recreationally in Colorado is
likely to be involved with are latitude / longitude and UTM (Universal
Transverse Mercator). In setting up
a GPS for use, you'll need to select a coordinate system.
For
example, here are the coordinates of Pikes Peak given in latitude / longitude
and UTM.
Latitude / longitude: 38°50.46
North, 105°2.64 West (Datum
WGS 84 (NAD 83)) (i.e. 38 degrees, 50.46 minutes North of the equator).
UTM: Zone 13
496181 Easting (meters)
4299133 Northing (meters) (Datum
WGS 84 (NAD 83)).
Both systems are commonly used,
but latitude / longitude does have a practical advantage. A common error in
dealing with GPS location fixes is to not know, forget, or get wrong, the datum
that was used to generate the location.
Roughly speaking, in central Colorado the amount of error involved in
making a datum mismatch for latitude / longitude is about 150 ft along an
east-west line. For UTM it is about 700 ft along a NNW-SSE line. That amount of
difference argues for using latitude / longitude.
For example, suppose somebody
gives you the coordinates for a trailhead for a hike you want to take.
In my experience, they will usually not give you the datum and if you ask
them for it, they'll look at you blankly. If
it's in latitude / longitude, and your GPS is not set to the same NAD/WGS datum
as the location was taken in, you'll be off by around 150 feet - usually no big
deal. If it's in UTM, then any datum mismatch would result in quite
a bit more error and in some cases might lead to problems in finding what you're
looking for.
Antennas, signal reception and getting a location fix.
Some
expensive professional use GPS units have big, heavy clunky external antennas
that can make the most of a poor reception situation. That's not what most
hikers would find appealing. For
recreational use GPS, Magellan Meridian units have built-in antennas that seem
to outperform other brands for the kind of challenges encountered in the
wilderness. A few GPS models have
plug-in jacks that facilitate external antennas, but this has the inconvenience
of an extra item and a wire to catch on twigs etc.
How are locations used with a GPS unit?
A waypoint is a single point location. It can be saved in the GPS unit's memory. Some examples are: the trailhead location for a hike; the fix for a mountain peak you will be climbing; the location of your house. On the GPS unit's map screen a waypoint can be seen along with an icon which denotes the position of the unit. As you are moving along you can see your current position in relation to the point you want to reach. Waypoints are sometimes created and saved to the unit's memory before leaving on a trip, and sometimes they are created in the field at a place whose location you want to record. Sometimes a series of waypoints are sequentially linked in order to form a route.
A route is a series of locations that are linked in sequence to describe a path along which you intend to travel. Usually, you decide on a way for getting from point A to B, "tell" the GPS you're creating a route, then input a series of locations in sequence. The maximum number of positions that can go into a route varies among GPS units, but the limit is normally several tens of them. You might construct a route which describes how to get from your house to the trailhead, and then another route that describes the hike itself. Obviously, several tens of locations will miss a lot of the detail of a convoluted path covering a long distance.
