These days we use lasers, computers and software to survey buildings. Although the tools may have changed, the basic principles have remained the same for thousands of years. Surveying, no matter the tools, is all based on the geometry that we learn in grade five. 
I found this illustration from 1782 of a 'Plain Table' as well as some figures showing its use. I don't have the text description, but here is how I interpret the method:
Points A and B are inaccessible points - the picture shows them on the other side of a body of water. They need to be located on a map relative to two accessible points C and D. Here's how you use the plain table to plot the locations.
First, place the table at location C. Using the ruler sights, draw lines in the directions of A, B, and D. Their point of intersection at c represents your current location at C.
Next, move the table to location D. Position the table so that the line c-d points at C. Use a scale to position the point d on this line.
Now use the ruler sights to plot lines from the point d in the directions of A and B.
Voila! You now have a scaled drawing of the relative locations of A,B,C, and D. You can use any of these points as a reference for locating more points and continuing the survey.
Today, a surveyor could use a laser and measure the distance to A and B from C across the river, and then measure the distance from D to C, A and B. But measuring angles or distance are both part of the same fifth grade technology - which has been around since ancient Egypt.
We have different equipment now, but the basic science is the same. Read how Building Measurement is done today.

