The 3x3 and 5x5 pattern does fit with the kind of survey I saw being done.
At each 3x3 square you would stand in the middle square and if something applied to all nine squares then you drew the correct shape in the bigger square. You would then examine each smaller square to record fine detail.
I think the outer 5x5 arrangement helps provide a reasonable number of measurements before you have to move your guidelines.
Each shape you draw indicates the reading. Perhaps one line for a low reading, a cross for slightly higher, then an outline, then a solid triangle, and finally a solid square. There are others here, an equals, some Y shapes, a teepee shape, and those lovely arcs.
The reason it all works is because the human eye can see patterns of particular shapes and colours very easily, especially if the shapes are more solid for higher readings. Note how the first image shows a kind of leaning back backwards question-mark shape in red triangles and similar shape in yellow running almost parallel to it.
If anyone from Photo Restoration is watching could you see if the very light writing on image 2 (looks like pencil) could be brought out in some way? If this is a survey of a garden they may write notes on some squares to explain some readings. 'Holly Bush' comes to mind.
A very good point on the felt-tips. These may not be that old but they are still fascinating.
Paul
