Estimating distance on the ground (Lat/Lon)

Being able to estimate distance on the ground is important for both navigation and field mapping. Maps measure the distance between two points “as the crow flies.” This means they measure horizontal distances, not slope distances. When navigating, if the land is flat this causes no problems, but when there are hills and mountains, distances measured on maps are going to be way off.

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There are various methods of determining distance along the ground. Since it is not usually practical to measure long distances on uneven terrain with a measuring tape, other methods such as pacing must be used. This section focuses on estimating distance using pacing and briefly mentions other methods for estimating distance on the ground.

Pacing

Pacing is one way to keep track of your distance when in the field. A pace is defined as the average length of two natural steps (a count is made each time the same foot touches the ground). Everyone has a different pace. Estimate your own average length of a pace by following the steps below.

Steps to determine your own pace

  1. Set up three pacing courses to ensure and accurate determination of pace length. Each course need to be 100 feet or 66 feet. One course needs to be on level ground and the other two need to be on a moderate slope and steep slope.
  2. Walk each course and count the number of paces (each double-step)
  3. Calculate your pace by dividing the distance measured (100 feet or 66 feet) by the average number of paces.
  4. Repeat this process a number of times to get your average pace.

To estimate the distance between two points on the ground, count the number of paces as you walk between the two points. Then multiply the number of paces by the length of your average pace. It is a good idea to have a system to record your pace count, especially over long distances.

Pacing is not exact and one has to compensate for the following:

  • Pace can change based on steepness of slope; typically, pace lengthens down slope and shortens upslope.
  • Walking into strong winds causes the pace to shorten; walking with a tail wind causes it to lengthen.
  • Soft surfaces such as sand and gravel tend to shorten the pace.
  • In wet, rainy, or icy conditions, the pace tends to shorten.
  • A paced distance may vary from a map distance because land surveys are based on horizontal distances, not slope distances, especially in uneven or rough topography.

Other Methods to Estimate Distance

There are other methods to estimate distance such as using your vehicle odometer and visual comparison. The odometer works great if you need to measure distances where roads are located. Occasionally you will encounter a stream too deep to wade or a slope too steep to cross, and the distance must be estimated. One way to do this is visually compare it to a known distance, such as the length of a football field.

 

**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.