Basics: Distance estimation
It's not enough just to know the direction of an object, we also need to know the distance from object to another. Distance estimation is a critical skill. Guidelines for estimating distances out to 500m are largely based on the appearance of people and familiar objects. There is a very good reason for this. Distance tables like this are core to any basic infantry army training manual to establish the range of the person you have to shoot... Distance estimation by appearance of objects is based on the premise that we are familiar with the ways things look, and we simply need to associate these appearances with measured distances. The table below provides a guide, but it is up to the individual to practice. This is pretty easy - given a known position, and a map, you can see and note how objects and landscapes look at different distances, as measured from the map.
Distance estimation by appearance of objects
||Appearance of familiar objects
||Facial features of people are clear; Clothing and other details clear; hair color recognisable
||Facial features of people blur; Clothing and other equipment details identifiable; individuals in a group identifiable
||Facial features not recognisable; Clothing and equipment color distinguishable
||Body outline of people and livestock is clear; Remaining detail is blurred
||Body outline of people begins to taper and head is indistinct from shoulders; Stock types (sheep, cattle) distinguishable; Large manmade features such as pylons and poles can be distinguished; Vegetation changes can be distinguished
||People identifiable as small indistinct objects; Different vegetation types distinguishable; Livestock recognisable; Buildings can be seen; Trig points can be seen; Rocky outcrops recognisable; Cars can be seen
||Trees can be identified; Livestock can be seen; Houses can be seen; Power lines can be seen
||Large buildings can be seen; Conspicuous hill features can be seen; Power lines can be seen
Factors altering distance estimation
That all seems well in theory, but this is based on the assumption that weather conditions and visibility are clear. There are other factors that alter how objects appear.
Lighting. On a bright day or with the light behind you, objects will appear closer than they are. This is particularly noticeable when the object you are estimating the distance to is well contrasted with the background. For example, a light-colored rocky outcrop on a dark green background will appear closer.
Perspective. A partially exposed target such as a hilltop behind a spurline in front will appear farther than it is. Similarly, a feature such as a small knoll will appear further away if surrounded by larger objects like high features.
Slope. Upward sloping terrain gives the illusion of shorter distance, whereas downward sloping terrain gives the illusion of greater distance.
Dead ground. If you cannot see all the intervening space between you and the object, it will appear to be closer. Conversely, smooth and continuous terrain such as sand, water, snow, grassy pasture, will make objects look further away.
Observer position. If you are sitting or lying (especially you shooters out there), objects will appear further away.
Aids for estimating distance
Unit of measure. If you can see all the intervening space between you and the object, you can mentally estimate how many units of a familiar length eg. a football field (100m), will fit in between you and the object.
Halving. This method is good to 800 meters. Estimate the halfway point between yourself and the object, then use the other methods to estimate that distance. Double that estimate to get your distance. This method works on the premise it is easier to estimate shorter distances.
Bracketing. To bracket simply identify the shortest and the greatest possible distance the object is from you. Your estimate is then midway between the two. This actually works pretty well to get in the ballpark of the true distance.
Unit average. Where would we be without good old democracy? If there is more than 1 person in the group, get everyone to make their best independent estimate and take the average. Maybe you can ignore the estimate from Uncle Clitus who is always measuring things relative to the length of his 1951 prize-winning marrow.
We move in a vector. A vector has two components – distance and direction. Compasses provide direction, but we need to know how far we are travelling. The easiest way to do this is to count paces. First, we need to work out how long a pace. This is easy. Count the number of paces required to cover 100m. There is, as always, a 'but'. A pace is defined as each time your left foot (or right - but not both!) touch the ground. The number of paces required to cover the distance will depend on your load and the ground. I cover 200m with 150 paces (actually, it's about 7 paces per 10m for me, but for convenience, 15 paces per 20m is easier to count). This is at patrol pace with a patrol order load (say, 15kg). For non-military types, this is a sedate pace of 2km/hour in open country or 1km in close country. We like to do things at a leisurely pace. Unless someone has the audacity to start shooting at us of course. I make different adjustments based on experience in different conditions.
Of course we need to count the paces. Various methods have been suggested such as shifting pebbles from one pocket to another with a certain number of paces (15 paces or 20m in my case), or pulling on a knotted piece of string and counting the number of knots. I just use a counter and click it every 15 paces. Presto, distance covered in 20m increments.
Unfortunately the horizontal distance we measure from the map, and the distance we actually cover when there is any appreciable slope are different. We need to correct our paced distance to the horizontal distance.
Slope correction is based on Pythagoras' theorem – the hypotenuse is the square of the sums of the squares of the sides of a right angled triangle. In other words the distance you walk is √(Map distance2 + Change in height2)
This is most easily expressed as a percentage of map distance. For example, you measure the distance from A to B as 200m, but you change height by 40m. How far do you actually walk? Well, 40/200*100=20. So the percent slope is 20%. Add 2% of 200m, and you will actually walk 204m. Wow! Make sure you have an extra bowl of cornflakes that morning!
However, let's say you measure a 500m map distance that climbs 300m ie. A slope of 60%, then the extra distance walked will be 500/100*16.6=83 so you will walk 583m. This starts to make a difference. Now I'm not in the habit of calculating square roots in my head - sorry to disappoint you folks. I know my life is pretty ordinary, but it's not quite that desperate. Yet. Here's a convenient table to use (provided in the downloads ):
Of course this has other implications. Imagine what happens to your pace length while climbing up or down a 80% slope, combined with our natural tendency to take the easy way up or gentle way down a slope. Animals do not walk in straight lines. Next time you are walking across farmland, look at where cows walk up and down hills. I strongly recommend jotting down your pace count under different slope conditions, different country (Open, Close, and Heavy eg. dense undergrowth, snow) at night and day, and slipping it in your Vueetuee. You will be surprised at the difference. I have a template in the downloads section. For example: