Fortification Friday: To a different dimension we go … for the plan, or trace of the works

Thus far in the discussion of principles of field fortification as taught by Mahan, I’ve focused on the components of the profile, and essentially the fortification as seen in the vertical plane.  But we know that fortifications exist in three dimensions.  So let us build that out further by considering the horizontal.  In Mahan’s own words:

The profile shows only the outline of the intrenchment in elevation, and by itself is not sufficient to point out the relative bearing of all the parts.  A plan, or trace, which exhibits the direction of the different lines of the parapet, &c., is, in conjunction with the profile requisite for this purpose.

Stealing a line… I love it when a plan comes together.

Fort Johnson

Or maybe…  remember these from 150 years and a couple more summers ago?


Yes, those are somewhat the antithesis of Mahan’s field fortifications, being the siege lines in the advance on Battery Wagner.  These were fortifications constructed by the attackers to shield from the assailing missiles fired by the defenders.  So the same rules applied.  Recall the profile lines are annotated in the drawing in the format “letter-letter+prime” – or o-o’ for example.  That’s how the plan or trace (which I prefer) worked along with the profile to provide a proper drawing of a three dimensional work.

The purpose of the trace is to demonstrate how the parts of the fortification work together in order to provide the defensive arrangements.  In that arrangement there were “parts” which provided that sum total of defense:

The plan of intrenchments, in general, should be so arranged as to procure a mutual defense of the parts. To effect this, certain parts are thrown forward toward the enemy, to receive his attack; they are denominated advanced parts; other portions, denominated retired parts, are withdrawn from the enemy, and protect by their fire the advanced parts.

Keep in mind that advanced parts and retired parts was not necessarily “front line” and “rear line” but rather described the orientation of the structures in relation to the enemy.  Let’s turn to Figure 2 from Plate 1 of Mahan’s Field Fortifications to illustrate.


Of that trace, these are advanced parts:


And these are retired parts:


Mahan continued with an explanation of the lines within those parts:

This arrangement naturally indicates that the general outline of the plan must present an angular system; some of the angular points, denominated salients, being toward the enemy, and others denominated re-enterings, toward the assailed.

So we have (some of) the salient lines:


And re-entering lines:


That’s the basics of a trace.  From here, we will take up the next lesson to consider, from that trace, the lines that become faces, flanks, and curtains.  We also need to consider the angles which become critical when assessed against the area covered by the defender’s guns.  All this builds up to the elaborate arrangements that use fancy words like redan, lunette, and redoubt.  Oh… and the star fort.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 3-4.)

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