Orienteering - Part 1
by John Mindock
Many books have been written about orienteering - this series cannot cover every facet of the subject. The emphasis will be on those aspects that SAR personnel would most likely utilize on missions.
Finally, there is no substitute for field work. The best usage for this series is to attempt to comprehend the subject matter, then perform the suggested field exercises.
This first lesson describes salient features of topographical maps.
Topographical MapsThe word 'topography' refers to the 'layout' of the land ( hills, valleys, cliffs, etc.). A topographical map (topo, for short) depicts those aspects of the land's surface by using contour lines, colors, and other devices. The common topo is known as a 7.5 minute map, because it depicts 7.5 minutes of latitude and 7.5 minutes of longitude. The scale on a 7.5 minute map is 1:24000, roughly equating to 0.1 miles of terrain per 1/4 inch on the map. Another name used often is a 'quad', referring to the quadrangular shape of the map. Topos are named after a significant feature which they encompass (I.e., the Sandia Crest quad).
Most topos were developed in the 1950 - 1960 timeframe, and revised in the mid-1970's. This often results in mis-representation of current features.
Contour LinesPicture a set of imaginary flat horizontal surfaces ('planes') that are parallel to one another, twenty feet apart, and slicing through the land. The contour lines shown on the topo are the intersection of the land with any of those planes, so that the outlines scribed by the lines provide a bird's-eye representation of the terrain.
The distance between adjacent contour lines is called the contour interval. Some maps use forty-foot contour intervals, while others use twenty. There is a phrase on the bottom margin of the map which states 'contour interval xx feet'. For the remainder of this series, we'll assume a contour interval of twenty feet.
If an area has many contour lines, the land rises upward more steeply compared to a place where there are few. With practice on a topo, one can become accomplished at envisioning the type of terrain depicted, recognizing features such as cliffs, ridges, valleys, arroyos, and saddles.
- Brown - contour lines
- Blue - places where water might occur (dashed if water is intermittent)
- Red - surveyor's markings (primarily land 'sections')
- Green - areas where tree-like vegetation existed when the map was made
- White - areas where the vegetation is smaller than trees (but not necessarily barren)
- Purple - modifications to the original map as a result of subsequent surveys
- Black - man-made things (trails, buildings, minor roads, fences, power lines, etc.) Black is also used for county lines, labels on map features, and other items.
Typical Land FeaturesContour lines for typical land features appear as follows:
- Hill - a series of misshapen concentric ovals, progressively smaller as altitude increases.
- Ridge - similar to a hill, except the 'ovals' are more elongated in two directions, like cigars.
- Cliff - a number of lines very close together on one side, with fewer lines on the other.
- Spur - a set of misshapen concentric triangles, progressively smaller as altitude increases.
- Canyon - a succession of 'V-shaped' lines progressing upwards towards the points of the 'V's.
- Saddle - a set of lower-altitude lines with hills/ridges on either side.
- Valley - a large area with few contour lines and higher features on either side.
- Peak - the top of a Hill, often with the uppermost point depicted as an 'X' , and that point's altitude nearby.
GridsGrids are man-made schema devised to provide reference lines/points for map features. The most interesting to SAR is the UTM (Universal Transverse Mercator). For this document, suffice it to say that UTM's are a set of numbers representing meters from the equator and meters from one of a group of 'North/South' lines. Although Latitude and Longitude are depicted on the map, their scale is usually too large for field personnel to use accurately.
Exercises - Orienteering Part 1
- Why are topos also called 7.5 minute maps?
- About how far (on land) is a distance represented by 3.75 inches on a topo?
- What are two differentials in altitude that contour lines often represent?
- How can one recognize steep areas by looking at a topo?
- What features are represented by the various colors on the topo?
Field ExerciseGo to a valley for which you have a topo. Locate nearby peaks, hills, saddles, etc. and compare what you see to the contour lines on the map. Then go to a high point and notice the appearance of the same features from the new perspective. Back to the Minilesson Page
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