Follow Us!
Archive for the ‘Map Reading’ Category
Determining a Back Azimuth!
The Opposite Direction!
Back Azimuth: A Back Azimuth, simply put, is the opposite direction. There will be times, in map reading, when you’ll need to determine the back-azimuth of a direction. This is used in Resection, which is a map reading technique used to determine your location when you’re not sure of what it is.
To obtain the back-azimuth for a direction/azimuth, you would add 180 degrees if azimuth is 180 degrees or less, or subtract 180 if azimuth is 180 degrees or more. The back-azimuth of 180 degrees (due South) would either be 0 degrees or 360 degrees (due North). For mils, if azimuth is 3200 mils or less – add 3200 mils; if azimuth is 3200 mils or more – subtract 3200 mils. The back-azimuth of 3200 mils (South) is either 0 mills or 6400 mils (North).
Care must be taken when converting azimuths as determine by a magnetic compass or grid map. A simple mathematical error/mistake could create terrible consequences.
Find Your Unknown Location From Two Known Points!
Determine Your Unknown Location!
Resection: This is the process by which you can determine your unknown ground/map location from two known/identified locations on the map. It is critical that the following steps be performed in sequence:
- Orient the map using the compass;
- Identify two or three known distant locations on the ground and mark them on the map;
- Measure the magnetic azimuth to one of the known positions from your location using a compass.
- Convert the magnetic azimuth to a grid azimuth;
- Convert the grid azimuth to a back azimuth. Using a protractor, draw a line for the back azimuth on the map from the known position back toward your unknown position;
- Repeat 3, 4, and 5 for a second position and a third position, if desired;
- The intersection of the lines is your location. Determine the grid coordinates to the desired accuracy.
Accuracy will depend on your ability to use a map. Compass, protractor, and follow the steps listed above.
Find Unknown Point From Two Known Points!
Remove the Guess Work!
Intersection: Locating an unknown point on the map from two known/occupied points on the ground. This map/compass method of determining a location on the map would be used when accuracy is desired and the unknown point is not accessible. The following sequential process would be duplicated:
- Orient the map using the compass;
- Locate and mark your position on the map;
- Determine the magnetic azimuth to the unknown position using the compass;
- Convert the magnetic azimuth to a grid azimuth;
- Using a map protractor, draw a line on the map from your position along this grid azimuth.
- Move to the second known point and repeat steps 1, 2, 3, 4, and 5.
- The location of the unknown position is where the lines cross/intersect. on the map.
- Determine the grid coordinates to the desired accuracy.
Accuracy will depend on your ability to use the map, compass, protractor, and follow the steps listed above.
Angular Measurement!
In Both Mils and Degrees
A thorough understanding of Angular Measurement and its components are sometimes critical to determining the correct Declination Constant data. This data is normally measured in degrees or mils, and is what you will either add or subtract from the base Direction/Azimuth to change it from grid to magnetic or magnetic to grid.
a. Degree: Most common unit of measurement and normally expressed as (o). Its subdivisions are minutes (‘) and seconds (“):
- 1 degree = 60 minutes;
- 1 minute = 60 seconds.
b. Mil: Normally expressed as mils or Mils. Most compasses and map protractors have scales in both mils and degrees. There are 6400 mils in a circle as compared with 360 degrees. The conversion ratio is 17.78 mils to 1 degree.
If accuracy is of utmost importance, the smaller form of angular measurement should be used. Knowing is not enough, you must have a quality map, a good compass, protractor, and a sharp pencil.
Lines of Direction!
Direction/Azimuth in Degrees!
Direction plays an important part in the process of a person being able to determine the data needed to navigate solely with a map/compass.
This Direction/Azimuth of travel is measured from a stationary point using either Grid North or Magnetic North as a base-line. If a map and a coordinate scale are used to determine the Direction/Azimuth – it would be a “Grid” reading. If the source of the Direction/Azimuth is a compass, then it would be a “Magnetic” reading.
As previously discussed in Blog entry: Map Declination Diagram, when taking Direction/Azimuth data from a map and wanting to follow it with a compass, the reading/data must first be converted from Grid to Magnetic. The reverse is true when taking data from a Magnetic Compass and wanting to plot it on a map (first change the reading/data from Magnetic to Grid).
From Map to Ground Distance!
Convert Measured Map Distance to Ground Distance!
Determining and converting map distance to ground distance is actually fairly easy. The scale of the Topographical Map will tell you how much 1-map unit of measurement (inches, centimeters or even feet) is equivalent to an x-number of the same type unit’s of measurement on the ground. Example: If the scale of the map is 1:50,000 – then, 1-inch on the map would be equivalent to 50,000 inches on the ground or approximately 4,166 feet; 1,388 yards; 0.79 miles and etc.
As referenced above, each topographical map has Graphic (Bar) Scales that can be used to measure and convert map distances (straight line and road) to ground distance. The Graphic (Bar) Scales will normally give you a mixed conversion in both metric and non-metric measurements, but, sometimes only one or the other.
Critical to your Map Reading and Land Navigation skill is an ability to be able to convert map-to-ground or ground-to-map distances. This may require the conversion of US-to-Metric or Metric-to-US data.
Location by Grid Coordinates!
Location by Grid Coordinates
In a city, it is quite simple to find a location. The streets are named and the buildings have numbers; but, in undeveloped areas, it’s not so simple. There is a method/process for determining and reporting positions or locations using a topographical map.
Superimposed on each Topographical Map are a series of numbered North-South (N-S) and East-West (E-W) grid lines. Where these N-S and E-W grid lines intersect forms a grid square. Each grid square is numbered using the applicable N-S and E-W grid line numbers. Each grid square is 1,000×1,000 meters.
As depicted in the diagram above, the key to determining the grid coordinates for a position/point on a map is to read right and up. Starting in bottom right corner of the map – read right to the N-S grid line closest (but still left) of the marked position (14); then estimate the distance in tenths (0-9) that the position is to the right of the identified N-S grid line (2); next, read up to the E-W grid line closest but still at the bottom of the marked position (84); next, estimate the distance in tenths (0-9) that the position is up from the identified E-W grid line (7). The grid coordinates to the position you have identified are: 142847. The US Military uses this Map Reading method/procedure extensively to identify locations on Topographical Maps.
Accuracy Is Important!
Magnetic to Grid - Subtract / Grid to Magnetic - Add
We have determined that a topographical map’s Declination Diagram establishes the angular relationship between true north, grid north, and magnetic north. This relationship becomes extremely important when taking information off the grid map and wanting to apply it with a magnetic compass.
When determining a direction/heading (azimuth) in degrees from a map and wanting to apply/follow it with a compass, it gets tricky. You must first convert that azimuth from grid to magnetic, and then apply it for use with the compass. Of course, the reverse is true when you want to take an azimuth from the compass and plot it for use on the map.
The Declination Diagram will be used to determine exactly what that conversion will be in Degrees. There may be a significant annual change to the Mean Declination data, if so, apply as directed. However, the most recent map/data should be used. This process is extremely important to map reading/land navigation accuracy. Each degree your off will be equal to 17.8 yards on the ground at a distance of 1,000 yards or approximately 160 yards at 5 miles – Accuracy is Important!
It’s All In The Color!
Map Color Representation!
To a large extent, map reading skill is based on an ability to look at a Topographical Map and be able to visualize what is being represented on the ground. Key to this capability is to know and understand the basic map colors and what each color represents:
- Black – Cultural (man-made) features such as buildings, secondary roads, spot elevations, and all labels.
- Blue - Water features such as lakes, rivers/streams, swamps, and drainage.
- Green- Vegetation such as woods, orchards, and vineyards.
- Brown - Elevation and relief features such as contour lines and possibly cultivated land.
- Red - Primary/main roads and possibly cultural boundaries.
- Other – As deemed appropriate and explained in the map’s marginal information.
These basic colors are not only exclusively used in Topographical Maps but also applicable to most other travel maps. A thorough understanding of map colors and their respective representations will enhance and compliment your map reading capability/skills.
Map Margin and Symbols!
Map's Margin/Symbols!
The key to learning how to use a Topographical Map is being able to find the information needed in the Map‘s Margin. Once you’ve found, read, and understand it, you need to be able to take that information and apply it using the applicable Land Navigation/Map Reading skills.
However, some of the information you’ll read in the margin is “nice to know” but will have little bearing on your basic navigational abilities. This blog entry will only identify/locate those map margin items felt to be critical for you obtaining a basic map reading/land navigation skill level.
- Scale (bottom center) – describes what one unit of measurement on the map is equivalent to on the ground (same unit of measurement (1:10,000)).
- Elevation Guide (lower right, ( not shown)) – provides rapid recognition of major land forms.
- Declination Diagram (lower right, (partially shown)) – indicates the angular relationship between true north, grid north, and magnetic north.
- Bar Scales (bottom center, (not shown)) – scales used to convert map distance to ground distance.
- Contour Interval (bottom center, (not shown)) – used to determine vertical distance/elevation.
- Legend (lower left, (partially shown)) – illustrates the symbol depiction of prominent map/man made features.
To further increase your Map Reading/Land Navigation knowledge, there are numerous subject books that can be reviewed at the local library or commercially obtained.
Most Commented