Categorization of Intervisibility Lines

"Why should I care about these IVL things?!"

Hey! I heard that! Here's why!

When maneuvering armored vehicles over open terrain against an enemy that could shoot back, soldiers quickly found out that they could exploit the subtle folds in the ground to gain a decisive advantage. Whoever got to the position first could either use it as a defilade and shoot the advancing and exposed enemy, or they could stop short in a hasty reverse-slope defense and spring an ambush. Because this was a subtle fold in the ground it wasn't obvious on the map. Before long, soldiers were recording these locations for future use, giving birth to the first "IV Line Overlay". The name seems to have come out of the National Training Center at Ft Irwin, California in around 1987.

Although commonly associated with the vast open expanses of desert battlefields, the concept is often useful in other environments and circumstances.

Introduction

Ever ask a Topographic Engineer to help you find an Intervisibility Line? No? If you are reading this in a Division, take a minute and call your Topo Engineer. Ask him or her for their " Intervisibility Line Overlay" for your Post or your unit's favorite training area. Go ahead, I'll stop writing for a minute and wait while you make the call....

What happened?

Did they explode over the phone? Did you get a lecture about how Intervisibility Lines don't exist?

Were you called Ugly Names?!

To hear soldiers talk you would think that Intervisibility Lines are tangible, physical object behind which to hide. To hear a commander shout "Hey S2 - gimme some EYE VEE LINE locations we can use!" you might think that they were a SIXTH type of terrain feature discovered since your last map reading class.

Intervisibility Lines (IVLs) are not terrain features, but an effect of terrain on observation. While Terrain Features are absolute and can be readily distinguished independent of the observer’s location, IVLs are relative. Topographic engineers dislike the concept of IVLs because of this relative nature. Combat Arms soldiers insist on using them because they describe a very real phenomenon, and are extremely useful in describing terrain from the Observation & Fields of Fire standpoint.

The problem is that not only have IVLs not been adequately defined, but they are not all the same due to the great variety of terrain and their relative nature. This article therefore aspires to achieve three worthy goals:

1) You got to meet your Topo Engineer, perhaps for the first time. (You DID make that call, didn't you?!)

2) It presents a definition of IVLs that might even satisfy your new topo friend.

3) It presents some methods for categorizing IVLs that will be useful for you in describing them to commanders, staffs, and other soldiers.

Defining the IVL

Intervisibility is the ability to to see from one object or station to another. (FM 5-33, Terrain Analysis, p 7-1)
Line of Site (LOS) is an unobstructed view from point A to point B. (FM 5-33, Terrain Analysis, p 7-1)
Observation (as in the "O" in OCOKA) is the set of points which have LOS from one specified location. In other words, what you can see.
Field of Fire (the other half of "O" in OCOKA) is the set of points which have LOS from one specified location out to the maximum effective range of a specified weapon. In other words, what you can shoot. The rest of the points are called "deadspace".

Intervisibility is the general concept. Line of Site is the concept of Intervisibility applied to two points. Observation is the concept of LOS applied to One Point in relation to All Other points. Field of Fire is the concept of Observation limited to a specific linear distance.

Definition of InterVisibility Line
A relative, localized, pattern of limitations on observation, caused by (often subtle) variations in terrain elevation.

Relative: Regardless of perspective or location, a hill is defined by concentric contour lines and remains a hill. This hill is Pikes Peak... that hill is a pitcher's mound. An IVL only exists in a specific frame of reference in relation to an Observer and the Observed. Move the observer from a fighting position to a tank, and the IVL changes. Put him in a helicopter and it might disappear. Change the Observed from dismounted Infantry to tanks and the IVL likely changes. This characteristic of IVLs makes the Topo engineer wince.

Localized: Given a fixed frame of reference, an IVL can be assigned to a specific geographical place. This characteristic keeps the concept of IVLs alive, and results in the infamous "NTC IVL Overlays" that Infantry/Armor commanders continuously refine.

Pattern: Given a fixed frame of reference, and a defined location, the IVL restricts observation in certain, predictable ways. This is generally known, but the patterns have not been described beyond the term "Line". Not all patterns are linear.

(Often subtle) Variations: The Observers/Observed are usually 1-2 meters in height. Map contours are generally 10 or 20 meters. There are many terrain variations that are less than 10-20 meters in height, and are not obvious unless they happen to pass through a contour interval. When these variations become large in relation to the observer/observed, they may block observation and constitute an IVL. The only upper limit in defining IVLs is common sense and usefulness. Yes, the Huachuca Mountains lying south of Ft Huachuca constitute an IVL for those looking from the Intelligence Center south into Mexico; but why waste time marking the obvious? It is that subtle fold in the ground, which will hide the tank company, that will stop your attack cold.

Components of an IVL

Shape | Engagement Distance | Defilade Quality

Shape

IVLs tend to fall into one of the four shapes illustrated below. Although contour lines are used to illustrate the basic shapes, keep in mind that the IVL is not a terrain feature.

	Type I - Blocks LOS uniformly along a line to the front. This is what is commonly visualized by "Intervisibility Line". Although portrayed as a long hilltop, in reality this is usually a subtle fold in the ground. Imagine moving your eye across a flat table - the table's edge is a Type I IVL.
	Type II - Blocks LOS more to one side than the other. In the diagram, there is no LOS restriction on the left-most arrow. In fact, to an observer at "A", there is no LOS restriction at all! Imagine walking around the side of a hill while maintaining the same elevation. As you walk, the edge of the hill to your front becomes a horizon. A friend standing at the bottom can see both you and what lies ahead of you over your localized horizon.
	Type III - Blocks LOS directly ahead, but allows LOS to either side. The drawing depicts a tank approaching a small hill. If the tank continues to steer directly toward the hilltop, the Type III Point IVL will become a Type I Linear IVL. If the tank gets very close and veers around either side, the Point IVL becomes a Type II Sloped IVL. Strictly defining the transition point between the types of IVLs is not important... ensuring the tank crew understands the effects the terrain will have on their (and the enemy's!) observation, fields of fire, cover and concealment is extremely important!
	Type IV - Blocks LOS to the sides, but not to the front.

Note that the observer's size and location determine whether there is an LOS obstruction as well as its type. A pitcher's mound is a Type III IVL to a turtle at 10 meters, and may become a Type II if he chooses to skirt the edge or a Type I as he crawls over the top. The mound presents no LOS obstruction to people.

Engagement Distance

This is the distance at which you can expect to be able to engage the enemy. This is a relative value based on the forced involved. If you are in an armored unit fighting an armored unit, and both are in the open, the 2 meter height is acceptable. If armor is attacking a dug-in defense, then the dug-in side must be calculated at 1 meter. Dug-in dismounts are less than 1 meter height.

The distance in meters that a 2 meter high object can see another 2 meter high object across the IVL. In other words, how close will the vehicles get before they can shoot each other?

Defilade Quality

Addresses the suitability of the IVL to provide a hull defilade position to an armored vehicle. Obviously, this is relative to the type of vehicle or force that might use the IVL for this purpose.

Labeling IVLs

	Draw the IVL at the appropriate place. Label with the roman numeral Shape, then the Expected Engagement Distance, then the Defilade Suitability. Omit elements when they are not known or needed.
	When an IVL changes characteristics, mark the approximate location where the change occurs and place the labels on either side.
	Sometimes the Intervisibility "Line" is better described by an area. For instance, the side of a large hill or ridge. Draw a polygon or circle and place the label on the inside.
	Sometimes the Intervisibility "Line" is better described by a point. For instance, the outlet of a pass or a small (19 meter) hill.

Finding IVLs

There are four major ways of locating IVLs, in order of preference...

1. The best way is to ask soldiers who have fought or trained on the terrain.

2. Walk over the terrain yourself. Ensure you fully understand the nature of the equipment you and the enemy will use. What is the height of the M1, M2, T80, BMP? The height of the gunsight? Works best if you take a friend, powerful binoculars, and lots of time.

3. Find IVL overlays of the area. They exist in abundance for NTC, but maybe not for where You will have to fight. Once you locate them, go back to techniques #1 and #2 in order to verify they exist where the overlay says they exist, and also to verify their characteristics.

4. Use topographic software like MicroDEM. Caution!

The DMA mapping data used by MicroDEM (and other such programs) is only accurate to the 10 or 20 meter contour interval... even though some programs will let you set the interval much lower.

These programs use a finite number of points and then calculate where to contour lines should go through interpolation.

They do not account for the effects of vegetation on LOS.

Aside from that, they are the next best thing to being there. These programs do not have a "Find IVL" function, so use the following technique.

a. Determine the area of interest.

b. Assume the role of the moving force. If both sides will be moving, assume the role of your own force.

c. Take a series of LOS snapshots. In MicroDEM, select Overlay | Weapons Fan from the pull-down menu. Between each snapshot, move 500 to 1000 meters in the direction of attack. The program will draw a series of lines radiating from your location... the lines indicate the terrain you can see. The lines break for deadspace.

d. After two or three snapshots, you will notice a few areas of deadspace that the snapshots had in common. The edge of the deadspace nearest the attacking unit is an IVL.

e. You must then study the pattern of LOS rays to determine the Shape. The length of the LOS rays as moving force moves through the IVL provides the expected Engagement Distance. You must walk the ground to get Defilade information.

Final Note

Quantitative vs Qualitative criteria.

The qualitative approach described above, using commonly understood or consistent definitions, will be sufficient for most tactical applications. It will certainly suffice in the early stages of verifying the usefulness and value of this system of categorization.

It was tempting to apply computer-aided geometry in order to add mathematical precision to IVL categorization. As far as we know, no attempt has been made to do so. You must therefore use your own best judgment to determine which of the above categories best describes an encountered IVL, then use the categories as a communication and analysis tool in developing tactical operations. Never forget to supplement map reconnaissance with ground reconnaissance. As stated before, a 20-meter contour interval leaves out a lot of detail, and the DMA data set referenced by MicroDEM, TerraBase, and other mapping programs is little better. Use no more precision than is required to get the job done. It is far more important to have a commonly understood labeling system to describe IVLs, than it is to attempt to add a facade of false precision.

Eventually someone may develop an automated system to locate and categorize IVLs from DMA terrain data.