This week’s post from weaponsman.com is a quick lesson in land nav. Like every week we share some of the best posts from our friend Kevin O’Brien, who passed away too young coming up on two years.
The best case is to have a map and a compass. If you have a general idea of the terrain you can navigate without either, of course. But if you have to choose one or the other, unless the map is complete crap, choose the map.
Why not choose a GPS? A GPS depends on things that you cannot control, including satellites (vulnerable to interception and destruction in wartime, and failure in peacetime) and the electromagnetic spectrum (vulnerable to jamming, meaconing, EMP and other QRM — manmade interference — and sunspots, areas of bad radio propagation (like iron-rich geological formations), and other QRN — natural interference.
Jamming GPS signals is child’s play, because (1) the frequencies used are fixed and published, and (2) a satellite is sending a very low-power signal from very far away.
A GPS also depends on something that has a knack for letting a guy down: batteries. GPS navigators and other smart devices are an update of the old pilot’s joke about a flashlight: something you put in your bag to hold dead batteries. (There are circumstances in which this joke is the very living soul of not funny).
What’s a Topographical Map?
A map is a graphic description of a physical place in (usually) plan view, meaning from an imaginary viewpoint overhead. There are innumerable kinds of maps. Planimetric maps are drawn to scale (of which more in a moment), show borders and boundaries, (usually) cultural features like roads, and coast- or water-lines. If you own a house or land, you have probably seen your lot on a planimetric map. A Mapquest street map page is a planimetric map (it’s also a thematic map, which is a kind of map that has a theme, naturally. Thematic maps can be planimetric, but don’t have to be).
A topographical map is a type of planimetric map that also shows the height of the terrain. How do you show the Z axis of the real world on a two-dimensional map? The convention for depicting height on modern topographic maps is to use isometric lines. That scary foreign word just means “same distance,” iso metric, see? So each height-depicting line on the map represents the same vertical distance as the others. This has some useful applications in the real world, which is where we want to use our maps, right?
It is the isometric lines or contour lines (so called because each line follows the contour of the land at a given height relative to mean sea level) that set a topo map (as we call them to save keystrokes) apart from other kinds of maps.
Unless you have occasion to work with very old maps, military topographic maps will be calculated in SI units, with isometric lines a fixed distance apart in meters and marked elevations (of benchmarks, hilltops and other significant Z Axis features) in meters as well, and distances and a scale in kilometers. In the US, topo maps made for civilian use will have these items marked in Imperial units — feet and miles.
Globally, topographical maps are very similar. Anyone who has used a British Ordnance Survey Map, USGS Map, or NATO military map can pretty much make the translation to the others no problem. Even a Russian or Chinese map is very useful (the Russians have always made superior maps). Even if you can’t read the language you can still see the terrain. The various grid systems used are not always interoperable, though. (We’ll get to that).
What’s On A Topographical Map?
There are essentially three things: the geological features, which include the basic shape of the terrain, things like hills, rivers, coastlines, and slopes; the cultural features, which are the things that grow on the terrain or that people build on it, like forests, villages, roads and railroads; and navigational and informational features, including various things that let you use the map.
A map can give you a good handle on terrain features, if you read the contour lines. This bit of instruction uses the topography of human hands to walk you through the most common terrain features. There’s a lot more the lines can tell you, and you pick it up instinctively sooner or later. For example, on any given map, since contour lines come only at one interval, the closer together the lines on the map, the steeper the terrain. You will notice that watercourses are always in the low point, and that contour lines form a V across the watercourse, with the narrow end of the V pointing uphill and upstream. Bodies of water and watercourses are geological features, and they are always depicted in blue.
This web page recycles government training materials meant to train soldiers to understand the association between the contour lines on their maps, and the terrain on the ground. It shows the basic terrain features; the hill above is one of them. (The page may have an annoying popup. Just dismiss it).
Cultural features include vegetation, usually shown as green, and anything humans built on the land, including roads, bridges, trails, railroads, power lines, structures, cities, etc. As a rule of thumb, geological features are more stable and useful for navigation that cultural ones. Barring Air Force intervention, a hilltop’s height isn’t going to change. The shape of roads and borders of towns change all the time.
Navigational and Informational Features
There are many of these, including the Legend, which describes the sorts of features you might see on the map; the declination diagram, which we’ll deal with in the next installment; the indicator of north (part of the d.d.) which is rather important; and information about the datum used (this is the mathematical description of the shape of the Earth that undergirds the navigational features) and the grid system. This is where we run into differences by nation and even by purpose of the map and its recency. Datums are occasionally updated and this means grids aren’t interoperable (some US maps still used the North American Datum of 1927 (NAD27) during our service, and other maps used WGS83 or another datum — a hazardous combination when you’re slinging lead and steel around). The Russians and their allies, for example, use a different grid system (Gauss-Krasovskiy) than NATO and their allies (MGRS, which is a superset of the Universal Transverse Mercator system). This gets interesting when you have lots of nationalities interoperating in one battlespace, but for most of you, the way to deal with this is:
- Check that everybody’s map has the same Datum and grid system.
- If not, get help! Your friendly SF intel sergeant can probably do MGRS to GK grid conversions, and your weapons guy can deal with artillery tuned with different numbers of mils in a circle.
Maps have grids that are set up for a military-type grid reference system, which should let you plot a point quite accurately, or alternately for latitude and longitude, depending on their intended use. Lat/Longs are hard to use in an on-foot situation, because in most of the world parallels and meridians don’t intersect quite squarely. The good news is, that even a map only gridded with lat/longs usually has ticks you can use to set up a UTM grid.
Grids are always read right and up. In map terminology, that’s easting and northing. How and why the grids are set up is part of every military map reading class, but do you know what? You don’t need to know it, any more than you need to know how a torque converter works to drive a car. Yes, it’s great to have knowledge in depth, but right now, you need knowledge you can use.
Some Homework if you want it:
Reading Topographic Maps, by the OK Geological Survey.