Simple Alidade

Taking a break from clothing and painting minis (yes, I'm still very slowly working on wargaming bits), I worked on a  small project yesterday that will help me increase, by practice, my understanding of 18th century surveying.

Alidade and Plane Table in use.

The alidade was a simple tool, used in conjunction with a plane table, to lay out lines in the field while surveying, thereby creating a map during the survey, as opposed to after.

Alidade and Cutting Board...er... Plane Table.

Most of the period alidades that I've found images of online are made of brass.  To keep things simple, I made mine from scrap wood leftover from other projects.

Sights folded.

The build was pretty quick.  The base and sights were cut to length.  I cut down one edge of the base to make it easier to run a pencil/pen along.  With the sights clamped together, I drilled and cut slots in the center.  The sight arms are attached to the base with small hinges, like original alidades, to allow for folding, making storage and transportation easier.  The sight lines are just thread held in place in tiny grooves at the top and bottom of the wood.

Sight Line.

Samuel Wyld, in his 1725 book, The Practical Surveyor, describes the use of the alidade (which he calls an Index):

"Assign on the Paper a Point, or stick a Pin at 0, (to represent the Hole in the Ground or present Station) to which Point, apply the fiducial Edge of the Index, and turn it about, keeping the Edge close to the Point or Pin at 0, till through the Sights you see the Hair cut a Staff or Mark, set up exactly in one of the Angles, as at then by the Edge of the Index, draw a Line from the Point 0 towards the Angle a, with the Point of the Compasses, without regarding the Length, so it be long enough."

I think this will make a good practice set up.

In other words, look through the sites toward a point, then draw a line on your paper.


Now I need to get back to work on a tripod and a chain for measuring.