Difference between revisions of "How to use a map and compass"

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(Aerial photos)
(Aerial photos)
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Determine your EXACT location on the map before you leave the parking area.  Scan the area to get a feel for the terrain and relate it to your map.  USGS maps often smooth cluttered contour data so you need to set your personal "data filter" at the same level.  Check your plan, take your bearing and start off.  Keep track of the distanced covered either by pacing or by checking off mapped landmarks as they appear along your route.  Always remain in map contact, that is, always know where you are on that map.  If you loose contact stop and regain it.  Look around for a landmark that is on the map, or return to the last spot where you were in map contact.  In easy terrain you may be able to simply follow a line of sight bearing to the cache.  If the terrain will not allow a straight line approach, then use the compass to keep the map oriented to the ground and navigate using contours and other mapped features to the vicinity of the cache.  Then use the bearings to the landmarks you selected during the planning stage to find the crossing point of all the lines.  The cache should be there.
 
Determine your EXACT location on the map before you leave the parking area.  Scan the area to get a feel for the terrain and relate it to your map.  USGS maps often smooth cluttered contour data so you need to set your personal "data filter" at the same level.  Check your plan, take your bearing and start off.  Keep track of the distanced covered either by pacing or by checking off mapped landmarks as they appear along your route.  Always remain in map contact, that is, always know where you are on that map.  If you loose contact stop and regain it.  Look around for a landmark that is on the map, or return to the last spot where you were in map contact.  In easy terrain you may be able to simply follow a line of sight bearing to the cache.  If the terrain will not allow a straight line approach, then use the compass to keep the map oriented to the ground and navigate using contours and other mapped features to the vicinity of the cache.  Then use the bearings to the landmarks you selected during the planning stage to find the crossing point of all the lines.  The cache should be there.
  
==Aerial photos ==
+
===Aerial photos ===
 
Add a good aerial photo to your data and you can just about walk to the cache and log in.  The photo adds data changes since the last USGS printing plus many things the USGS maps omit.  Trails, single trees, evergreens in deciduous forest, wood lot corners, stream bends, even the shadows of those elusive lamp posts, all show up perfectly.  Most photos are taken during the "leaf off" seasons so even the density of the vegetation can be seen.  Remember that in the northern hemisphere shadows cast during the time that mapping photos are taken will point from NW around to NE, never to the south.  This can be important, for instance, if you are using a long shadow of an evergreen to pinpoint the exact location of the tree.  If you assume the tree to be at the wrong end of the shadow, you may be off by a couple hundred feet.
 
Add a good aerial photo to your data and you can just about walk to the cache and log in.  The photo adds data changes since the last USGS printing plus many things the USGS maps omit.  Trails, single trees, evergreens in deciduous forest, wood lot corners, stream bends, even the shadows of those elusive lamp posts, all show up perfectly.  Most photos are taken during the "leaf off" seasons so even the density of the vegetation can be seen.  Remember that in the northern hemisphere shadows cast during the time that mapping photos are taken will point from NW around to NE, never to the south.  This can be important, for instance, if you are using a long shadow of an evergreen to pinpoint the exact location of the tree.  If you assume the tree to be at the wrong end of the shadow, you may be off by a couple hundred feet.

Revision as of 04:49, 12 February 2006

Long before GPS, people were using maps and compasses to find their way around. You may be surprised at what you can do with these two important tools.

Contents

Terms

Map

A topographical map

When we talk about a map in this article, we are generally referring to a good topographical map such as what can be found at Topozone.com or Mytopo.com. Road and street maps are pretty much worthless out in the woods. Topo maps have all sorts of information on them like elevation, water, land types, land marks, etc. These maps usually have enough features marked on them to guide a map reader to the vicinty of the cache. At scales of 1:10000 or 1:5000 an experienced navigator can get to within a few meters of a cache placement. (They also have degree or utm grids or hashmarks on them so you can actually compare where you are on the map to your gps coordinates, or find your gps coordinates if you know where you are on the map).

Compass

If you are using a standard base plate compass: Your compass will have a needle built into it that points to Magnetic North (red end), and is inside a liquid filled housing with parallel lines in the housing. The housing can be rotated 360 degrees. There will be a transparent base plate that has a directional arrow.

There are other types of compasses employing various sighting techniques for surveying or map production, but for navigation in conjunction with a map, a simple base plate model is fine.

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Techniques

Taking and following a bearing

To take a meaningful bearing you must know where you are currently located and recognize that location on the map. Of course you also need to know where you want to go and where that is located on the map. When you know these two things simply connect "where you are", with "where you want to go" by placing the compass on the map. Use an edge of the clear base plate with the directional arrow pointing to "where you want to go" to connect those two points. Now while the compass is on the map turn the housing until the lines inside the housing are aligned with *magnetic north. Lift the compass off the map, turn the entire compass (not the housing by itself) until the magnetic needle and N on the edge of the housing are aligned. The direction of "where you want to go" is now in the direction of the travel arrow on the clear baseplate. Sight out to the furthest distinct landmark in that direction and walk toward it. If you can maintain the bearing just keep repeating the sighting process. If the terrain does not allow walking in a relatively straight line then more map reading and less compass reading will be required.

  • magnetic north can be determined by looking on the border of the USGS sheet. It will vary with location and may not be totally uniform in an area. It is a good idea to put your own magnetic north lines on the map at home before going out or memorizing the declination at your favorite caching areas.

Determining your location on a map - Map Reading

Yes, it can be done without a GPS. Maps have literally millions of bits of data on every sheet. The trick is to use them effectively.

Planning

Find your destination's location on the map then plan on how to find it. Determine your starting point and look at the potential routes to the cache. Read the contour lines. Closer together means steeper, further apart means less steep. Consider the climb and the distance to be covered and make route choices based on your physical abilities, the weather, and your map reading skills. Look at the water features. Stream crossings can be dangeous, but streams often make good "handrails" to lead you to a specific landmark. Find two or three features near the cache location that you feel you can identify when you reach them on the ground. Draw and label magnetic bearings from these landmarks to the cache at home when you are not affected by weather, fatigue, or excitement.

Judging Distance

Know your pace. That is, know how many steps you take to travel a certain distance. Realize that uphill and rough gound will increase the number of steps and practice enough to know how much of an effect that is for you. Be able to measure distance on the map using the scale on the edge of the compass. Make your own scale of hundred foot increments on adhesive tape if your compass edge has no scale that suits your liking. You could use a football field to determine your 100 yard pace in perfect terrain.

Execution

Determine your EXACT location on the map before you leave the parking area. Scan the area to get a feel for the terrain and relate it to your map. USGS maps often smooth cluttered contour data so you need to set your personal "data filter" at the same level. Check your plan, take your bearing and start off. Keep track of the distanced covered either by pacing or by checking off mapped landmarks as they appear along your route. Always remain in map contact, that is, always know where you are on that map. If you loose contact stop and regain it. Look around for a landmark that is on the map, or return to the last spot where you were in map contact. In easy terrain you may be able to simply follow a line of sight bearing to the cache. If the terrain will not allow a straight line approach, then use the compass to keep the map oriented to the ground and navigate using contours and other mapped features to the vicinity of the cache. Then use the bearings to the landmarks you selected during the planning stage to find the crossing point of all the lines. The cache should be there.

Aerial photos

Add a good aerial photo to your data and you can just about walk to the cache and log in. The photo adds data changes since the last USGS printing plus many things the USGS maps omit. Trails, single trees, evergreens in deciduous forest, wood lot corners, stream bends, even the shadows of those elusive lamp posts, all show up perfectly. Most photos are taken during the "leaf off" seasons so even the density of the vegetation can be seen. Remember that in the northern hemisphere shadows cast during the time that mapping photos are taken will point from NW around to NE, never to the south. This can be important, for instance, if you are using a long shadow of an evergreen to pinpoint the exact location of the tree. If you assume the tree to be at the wrong end of the shadow, you may be off by a couple hundred feet.

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