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Archive for the ‘Map Declination Diagram’ Category
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).
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!
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