Archive for the ‘Map Reading’ Category
Minor Terrain Feature – Cliff!
Map/Terrain Association!
A Cliff is a vertical or near vertical feature. It is considered to be an abrupt change in the land structure. When the land is so steep that the contour lines converge into one, the last contour line will have tick marks pointing towards low-ground.
But, cliffs are also shown with continuous lines very close together and in some cases actually touching each other. This of course also indicates a very steep-to-vertical up/down-grade.
As stated previously, map reading proficiency is based (to a large extent) on a person’s ability to associate what they’ve seen on a map with what is found on the ground. This is such a degradable skill and must be practiced frequently.
Minor Terrain Feature – Draw!
Map/Terrain Association!
A draw is considered to be a Minor Terrain Feature. It is a less developed stream course than a valley and has little -to-no level ground or maneuver within its confines.
If you’re standing in a draw, the ground slopes upward in three directions and downward in the other direction. It might be considered as the initial formation of a valley. The contour lines depicting a draw are “U” or “V” shaped, with the closed portion of the “U” or “V” shape pointing up-hill or towards the the high-ground.
Critical to a person’s map reading proficiency is their ability to associate what they see on the map with what they find on the ground.
Minor Terrain Feature – Spur!
Map/Terrain Association!
As stated previously, critical to a person’s map reading proficiency is their ability to associate what they see on a map with what is found on the ground or vice -versa.
Major Terrain Feature – Depression!
Map/Terrain Association!
A Depression is a low point in the ground or a sinkhole. It can not be better described than a “Hole in the Ground” with high-ground on all four sides. Usually, only depressions that are equal to the contour interval or greater are shown.
On a topographical map, depressions are represented by closed contour lines that have tick marks pointing towards the low-ground.
As stated in other postings, it is critical for map reading proficiency that the using person be able to associate what is seen on the map with that observed on the ground.
Major Terrain Feature – Ridge!
Map/Terrain Association!
A Ridge is a sloping line of high-ground. If you are standing on the center-line of a ridge, you will normally have low-ground in three directions and high-ground in one direction with varying degrees of slope.
If you cross a ridge, at right angles, you will climb steeply to the crest and then descend steeply to the base. When you move along the path of the ridge, depending on the geographical location, there may be an almost unnoticeable slope or a very obvious incline. The contour lines forming a ridge tend to be “U” or “V” shaped with the closed portion pointing towards the low-ground.
Critical to your map reading proficiency is an ability to associate what you see on the map with what you find on the ground or vice-versa.
Major Terrain Feature – Valley!
Map/Terrain Association!
A valley is when you have fairly level ground bordered on the sides by higher ground. It may or may not contain a stream but will generally have maneuver room within its confines.
The contour lines indicating a valley are “V” / “U” shaped and have a tendancy to parellel a stream before crossing it. The course of the contour line crossing the stream or closed portion of the “V” / “U” always points up-hill.
Critical to your map reading proficiency is your ability to accurately associate what you see on the map with what you find on the ground or vice-versa.
Major Terrain Feature – Saddle!
Map/Terrain Association!
The saddle is a dip or low point between two areas of high ground. A saddle is not necessarily the lower ground between two hilltops; it could be simply a dip or break along a ridge crest.
If you are in a saddle, there is high ground in two opposite directions and lower ground in the other directions. It is normally represented as an hour glass or sometimes referred to as “two eggs” in a frying pan as seen from above.
Critical to your map reading proficiency is the ability to associate what you see on the map with what you find on the ground.
Major Terrain Feature – Hill!
Map/Terrain Association!
A hill is an area of high-ground. From a hilltop, the ground slopes down in all directions. It is shown on a topographical map by contour lines forming concentric circles.
The inside of the smallest circle is the hill-top. This is sometimes marked with an “x”, including the elevation above sea-level in either feet or meters (Bench Mark).
There are electronic devices available that will give altitude readings instantaneously.
Contour Interval and Terrain Features!
As Seen From Above?
Contour Lines are used to show map elevation/relief and usually depicted as a shade of brown. The distance between the Index Line and the Intermediate Line is called the Contour Interval. This interval is normally represented in feet or meters as stated in the map’s margin.
As shown below, the Index Line is solid dark brown; Intermediate Line is solid light brown. There may be a Supplementary Line, if so, it would also be light brown but broken. A Supplementary Line would be half the distance between the Index and Intermediate lines.
- ____________________ (Index)
- ____________________ (Intermediate)
- - – - – - – - – - – - – - – - – - (Supplementary)
These Contour lines form Terrain Features which permit a visualization of the earth’s surface as seen from above. The Major Terrain Features that could be represented are Hills, Saddles, Valleys, Ridges, and Depressions; or Minor Terrain Features, such as Draws, Spurs, Cliffs, Cuts, and Fills. Critical to your map reading proficiencyis an ability to associate what you see on the map with what you find on the ground.
Find Unknown Point From One Known Location!
Find Unknown Location W/Grid Azimuth and Distance!
Polar Coordinates: This method of finding an unknown location on the map requires an azimuth and distance from your position to the unknown location. It will be as accurate as you are in determining distance to the unknown location:
- Identify and mark your position on the appropriate topographical map;
- Using a compass, determine the magnetic azimuth from your position to the unknown location;
- Convert the magnetic azimuth to a grid azimuth and with a protractor, plot that azimuth on a straight line starting at your present position;
- Estimate/determine the distance to the unknown location from your present position;
- Using the applicable map distance scale, plot the estimated/laser determined distance on the azimuth’s straight line and mark it. This mark represents the previously unknown location.
This mode of determining an unknown location on the map will depend on your ability to use the map, compass, protractor, and follow the steps listed above.