MORE INFO on MAPS & NAVIGATION
You can read more about maps & navigation in this section:
- Understanding Maps (contour lines, map scales)
- Compass (true north)
- More on how does a GIS work?
- GPS- Who put it in space?
- More on the 3 segments of GPS
- More on how GPS works
- More on GPS Accuracy
- Tips for using your GPS receiver
- Helpful Links
- Or want to go to the Maps & Navigation intro page?
What are contour lines?
The distinctive characteristic of a topo map is that the shape of the Earth's surface is shown by contour lines. Contours are imaginary lines that represent a specific elevation (the distance of something from a given level). Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes. Each line on a map represents a specific elevation. All locations along that line have the exact same elevation.
Doesn't a topo map show more than imaginary lines (contours)?
A topo map shows more than contours. The map has symbols that represent such things like streets, buildings, streams, and vegetation. Each of these may have a color or a symbol. There are also many other features on a map that are shown by lines that are straight, curved, solid, dashed, or dotted.
The first features usually noticed on a topographic map are the area features, such as vegetation (green), water (blue), and densely built-up areas (gray or red).
From Near to Far: How to find Distance on a map
If, for instance, 1 inch on the map represents 4 miles (which converts to 253,440 inches ) on the ground, the map's scale is 1:250,000 (round the number 253, 440!). Here are some examples. You can see as the scale changes, the definition in the map changes.
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1:24,000 |
1:100,000 scale map |
1:250,000 scale map |
| 1 in = 2000 ft | 1 in = 1.6 mile | 1 in = 4 miles |
| 1 cm = 240 m | 1 cm = about 1 km | 1 cm = about 2.5 km |
| Map area is approx. 40-70 square miles | Map area is approx. 1568 - 2240 square miles | Map area is approx. 4580 - 8669 square miles |
Find a map in your class, and check out the scale!
COMPASS
Does a compass really point true North?
When you read north on a compass, you're really reading the direction of the magnetic north pole, not what the 'true' north is. Why? The compass needle is attracted by magnetic force, which varies in different parts of the world and is constantly changing.
You can find a diagram in the map margin that shows the difference (declination) at the center of the map between compass north (magnetic north indicated by the MN symbol) and true north (polar north indicated by the "star" symbol). Because the magnetic declination is computed at the time the map is made, and because the position of magnetic north is constantly changing, the declination factor provided on any given map may not be current.
HOW DOES a GIS WORK?
With GIS it is all about putting data together and finding the relationships between different information. What if you wanted to check out what marshes received the least rainfall? How would you start?
You would first take the location of the marsh--which could be the longitude and latitude. You could also take the elevation. You could then compare the rainfall that the marsh gets each year, with another marsh in another area. What if the marsh received a lot less rain than others? What could this mean? This could mean that the marsh may dry up. This could help environmentalist to figure out if something needs to be done to save the marsh. Therefore, GIS is helpful for making decisions.
GIS is also important because computers 
can take all kinds of statistics, location or map information and other types of data, and combine it together. This means GIS can help create new maps and make new predictions, that could not be done before!
GPS- WHO PUT IT IN SPACE?
The satellites were placed into orbit by the US Department of Defense (DoD) in 1978. The DoD’s original intention was to use GPS for military applications (e.g. identifying military targets). 
To ensure military enemies did not have access to the most accurate data, the GPS had Selective Availability (SA), which made the GPS signals less accurate (100-meter accuracy). In 2000, a US presidential order called for the SA to be turned off, thus all GPS devices could have accuracy up to 15 meters.
The US government invested over $10 billion USD to build the system. The cost of maintaining the system is about $400 million USD per year, including the replacement of aging satellites.
GPS is operated and maintained by the Department of Defense (DoD).The National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee manages GPS.
Overall management and control of the satellite constellation. Located at the Schriever Air Force Base near Colorado Springs, Colorado, USA. (
The user equipment segment, usually referred to as ‘GPS receivers’ are devices that can receive GPS signals (L-band) from the satellites in view. The GPS receivers can calculate your position, velocity and time.
Each point on Earth is identified by two sets of numbers called coordinates. These coordinates represent the exact point where a horizontal line, known as latitude, crosses a vertical line, known as longitude. When a GPS receiver is locked onto three or more satellites, it can use the information from the satellites to determine the coordinates of your location.
The calculation the GPS receiver uses to determine your position is called, trilateration, the determination of a distance from three points.
With this information, the GPS receiver can then calculate speed, bearing to selected waypoints, altitude, trip distance, trip time, distance to destination, sunrise and sunset time, and more!
The accuracy of GPS can be improved in a number of ways.
- Differential GPS (DGPS) improves accuracy by using an additional receiver at a fixed location nearby. This helps increase accuracy to within about 1 meter (3 feet).
- Wide Area Augmentation System (WAAS) improves accuracy by using a system of ground reference stations that receive signals from additional satellites. This information helps to correct signal errors. This helps to improve GPS accuracy to 3 meters.
- Local-Area Augmentation System ( LAAS ) improves accuracy, like WAAS, to correct signal errors, but this information is not from a satellite but a local source, as an airport.
TIPS FOR USING YOUR GPS RECEIVER
Have a GPS receiver of your own? Want some tips on how to use it? Read about it here.
- Keep your unit in 3D mode: some models are locked into 3D and do not permit you to toggle between 3D and 2D.
- Re-initialize your GPS if you've traveled several 100 miles since last using it: this will make acquiring a position fix faster. Some models automatically perform this function.
- Always verify you're using the correct map datum, elevation mode, north indicator, and coordinate system. Some models will default to the manufacturer's specifications when the unit is turned off.
- Keep the antenna pointed skyward for best reception.
- Always verify you're receiving enough signals (via the satellite status screen) before making critical navigation decisions.
- Always carry spare batteries.
- Practice your GPS skills in safe, familiar surroundings before going into the wilderness.
- Don't use the backlight function until necessary - it will drain your batteries.
- You may set your GPS to the UTM coordinate system & also set the navigational units to feet/miles. This will allow you to use the metric-based UTM coordinate system, but display navigational distances in feet/miles.
- Before going into the wilderness, it's a good idea to take a GPS waypoint at your car. This way, you'll always know the straight-line distance and direction to safety.
- Give your GPS waypoints intuitive names. For example, if hiking in Carson national forest, name them cnf1, cnf2, cnf3, etc.
- Confluence Project (fun for the true GPS addicts)
- Garmin
- Geocaching (The Official Global GPS Cache Hunt Site)
- Geological Survey of Canada (North Magnetic Pole data and diagrams)
- Google Earth (very cool site - maps, satellite images, find your school)
- GPS Overview (Univ. of Colorado)
- Having Fun with GPS
- How Stuff Works (How GPS Receivers Work)
- Magellan (GPS receivers)
- NYGPS (Community of Teachers Using GPS for Math, Science & Social Studies Instruction)
- TerraServer - USA (US maps)
- TopoZone (examine any of the USGS topographic maps for free, look up locations, map names)
- Trimble (GPS tutorial)
- Upsidedown Map Page
- US Federal Aviation Administration - FAA (excellent GPS resource)
- US Geological Survey (USGS)
- US National Air and Space Museum -GPS
- US National Geodetic Survey ( Use geodetic tool kit for magnetic declination calculators, other useful software, information on geoids, ellipsoids)
- US National Oceanic & Atmospheric Administration-NOAA (marine chart source)
- USGS Geographic Names Information System (GNIS)
- UTM Grid Zones of the World
