This is a picture of a medieval master mason, likely the chief architect of a cathedral, with a very important tool in the practice of medieval architecture, the large 'outdoor compass'


Another picture from a 14th century manuscript, depicting a king with his chief architect.

The giant compass is used to transfer measurements from the architect's plans to full scale at the building site. The procedure here seems to have been to take a few key measurements from the drawings with the small drawing compass, and set the the large compass to several times the opening size of the smaller one, which is then swung a number of times to yield the final measurement.
To do this you need a scale, say 1:150, this is the scale of the drawing. You then need to take two factors of this number, say 15 and 10. To get the full scale measurement, you might first use the small compass to make a line 15 times the size of the original on the drawing, then set the large compass to this size and swing it out 10 times to yield the full sized line. This procedure is subject to slight inaccuracies on the order of perhaps an inch or two in 50 feet.

Now I have this on the tool forum, because I have a question.

I wonder if it would be possible for me to obtain, or else have made, a giant compass such as this.

The following can be observed about its construction:

They are generally somewhere around 3 feet in height, made of metal (presumably steel)
They have at least 1 retaining arc to help hold their positioning, whereas contemporary drawing compasses generally had no retaining arcs. These arcs are necessary to keep the compass in perfect alignment as the weight of the legs would be too much for a set screw at the joint to handle.

It seems that the retaining arcs worked by means of a light hammer tap, by which they were driven into wedged openings and thus held the alignment by friction. Or alternately they may have been held into position by driving a wedge into the openings in the legs that they passed through. Perhaps a modern version could work adequately with set screws rather than wedging.
I would like to have such a tool because I think it would be very useful to me, and easier to use than a rope compass.

Hi DL, How bout some trammel points? You sure do like to step things off.

Or you could make huge proportional dividers.



1/15:1 Step off ten times. By the way these dividers are really quite neat for proportional design work. Oh yea and really quite expensive too.

The large compass has the big advantage of allowing you to stand up while using them. And most of all to me they are of great historical interest.

The method described is how it is thought they built churches and cathedrals, among other things, in the Middle Ages. The documentary evidence certainly suggests it, as does the mensural evidence. The system works, and works quite well. It gives the architect the means of scaling up his plans to create the building. This is important because it makes so that he knows his drawing will meet the specifications of the job.

What would have been done is that the person ordering a building would specify one specific measurement for a certain dimension of the building, in the case of a church or cathedral this usually meant that some important measurement such as the length of the nave was equal to 153 feet, 153 feet being the number of fish in Peter's net. The architect could then set that measurement on his drawing to something proportionately equal to that somewhere in the neighborhood of 1/150th or maybe 1/180th the size. This measurement would then be used as the basis for the geometry of the rest of the structure.

The most important thing is that this system allowed buildings to be built by people who were usually totally illiterate, and very innumerate by today's standards -possessing only the most basic skills of mathematics. It made it possible for the master to lay out plans and instructions for people who could not read, barely even add, and may not even have spoken the same language as him.

This would be of use for me today, because the systems of geometry I have developed work the same way. When I make a geometric design, I pick 1 measurement and use it as the basis of my entire geometry. I use some mathematics so that I can scale up my drawing with a measuring tape, but I would like to be able to take that out of the picture as well.

I doubt proportional dividers would work very well for making measurements on the order of several feet, perhaps 10 feet or more at times. I despise using trammel points on the ground, it's very disagreeable to my knees and back.

The large compass becomes very attractive when there is the prospect of swinging a lot of arcs...

I am interested in this tool as a building site tool that will allow me to leave my measuring tape at home...

To illustrate what I am talking about, I will use a real example.

For a building I have designed for myself using pure geometry I have come up with a measurement for the ceiling height that would be somewhat too short if using a perfect inches to foot system from drawing to real world. There are two solutions to this problem. The first is to measure out my master dimension when I go to make the full size plan by using a foot that is actually 13 inches long instead of 12. The second is to use the factorial 2 compass system, but just change my factor a little bit to make the building slightly larger. The current factor is 1:24, but I could just as easily make it 1:26 (which would yield the same dimensions as if I were to use a 13 inch foot)

If I would use the first solution, that means I would have to redo all of my geometry at full scale. If I would use the second system then I could take a lot of shortcuts when making the full plan.

Just for a quick example, as designed 3 inches on the plan equals 6 feet. Right now my ceiling height comes out to about 7 feet, which would be 6.5 feet with the frame taken into consideration. If I make it a proportion of 1:26 then the ceiling height becomes about 7.58 feet, which would be about 7 foot ceiling after framing (a little less). I am considering using a factor 27 system (9x3) which would make the floor plan of my building 13 1/2 by 27, instead of the current 12 by 24. I wouldn't mind those extra 76 1/2 square feet...

You may ask the question, why didn't I just make the walls higher to begin with? The reason is I did not like the way the proportions came out when I did, the building looked too high compared to its width. So I decided I would rather just scale up the width a little bit. the 12 foot width was the master dimension I used for my geometry, but it was just a guideline. I wasn't married to it so I didn't mind changing it to make things come out right. I had a couple feet to fudge with

24" dividers are usually available, I currently have one wooden set that has 16" legs, and one metal set that has 21" legs.

I don't have any that are as tall as the pictures show, but I'll watch for them.

In all my years of finding tools, I have not see a set this large. They may have to be custom made, which can be done.

I assumed they would have to be made, that's why I gave some technical information on how they operate. But I did have a slight hope that somebody supplied them.

I suspect 2' is about the maximum size you could manage without having to incorporate the arcs for support. The extra foot of length would make a huge difference too.

I doubt they have been used very much since the 16th century...

2 feet would be a handy size for marking off timbers and laying out joinery, but I am afraid it would be too small for laying out a building plan

Years ago, at a workshop event before a conference, we watch a pair of French timber framers layout a full size drawing on the floor. They used trammel points to draw large arcs. But that means you have to get down on your knees to do it....

Yes, the whole getting down on the knees thing is unattractive to me. I can see where it would be a lot more pleasant for many years to not have to do that

I am considering making an 'experiment' out of wood with metal points to see how well it works.

I can't imagine the set in the first drawing are made of iron. They would weigh 50 pounds.

It would make sense. The two retaining arms would be sufficient to ensure the wood doesn't flex or break in use. A nice wooden compass could be made quite attractively as well, with some nice decoration and so forth.

I might just look through my stock and find some nice straight grained wood for this project, some split stock I would think. Of course it would have to be hand tooled! It would just seem wrong to use power tools to make a Medieval compass...

What would be a good wood as far as stiffness and strength. It would need to be able to have holes in it and metal fasteners.

I will draw up some plans and see what I can come up with!

I've seen large wooden dividers at the Maison de L'outil in Troyes (http://www.maison-de-l-outil.com/) and even today compagnons still make their own. We have few modest 2' long sets we made here, with drywall screws filed down for the points.

I think dry beech or hickory would be good woods for this.