Measuring Things

US Coast & Geodetic Survey leveling party working in Atlanta, 1927

In the olden days, like before GPS, before you could make an accurate map real men had to go out and measure things.  This 'measuring' was called surveying, and it involved the extremely precise and accurate determination of the horizontal and/or vertical location of points on the ground known as survey control.  This survey control establishes the accurate framework upon which a map is built.  Horizontal measuring was called triangulation. Vertical measuring was called leveling.

The picture above comes from the US Coast & Geodetic Survey 1927 Seasons Report prepared by Captain E. O. Heaton (USC&GS).  It shows a topographic leveling party at work in Atlanta.  If anyone can figure out where in Atlanta these guys are working I'd love to know!  

A few things to note.  The fellow holding the umbrella is most likely a black local laborer hired to help the party haul equipment and provide general assistance.  The umbrella he's holding isn't to keep the surveyor from getting sunburned - it is to protect the instrument from direct sun and prevent glare when sighting through it.  To ensure accuracy survey parties often used umbrellas to shade their instruments and stabilize temperatures.  

The fellow squatting is a surveyor who is acting as the recorder.  He is writing down the readings being called out by the surveyor looking through the instrument.  The recorder's job was extremely important because he didn't just write down what the surveyor called out, he would do on-the-fly quality control checks on the values the surveyor gave him to ensure they were staying within the accuracy standards established for that particular survey.  If the recorder makes a single mistake, such as not catching an error in the surveyor's observations or by writing something down wrong (like inadvertently transposing a number or putting a decimal point in the wrong place) he could lose an entire day's work.  In my experience you wanted your  most meticulous guy and your best mathematician doing this job - perfectionists made good recorders.  The recorder is writing his notes down in a bound hardback book known as a survey field notebook.  That notebook would be turned in to the USC&GS at the end of the project and go on to become a part of the legal record of the survey.  I have no doubt that very notebook still exists in the archives of the USC&GS now held by the National Oceanic and Atmospheric Administration.  I'd love to take a peek at it!

(The job of recorder is one of those skills that has been replaced by computers.  Today's surveying instruments now automatically store the readings and calculate values internally.  The computer integrated into the survey level lets the surveyor know via a digital display if the readings are within the specifications set for the job.  It's called digital leveling.)

But what is the surveyor looking at?  Well, there are two people missing from this photo that make up the leveling party.  The surveyor is looking through the survey level at a stadia rod being held by another party member known as a rod man.  A stadia rod is a long pole marked off in feet and inches.  Behind the surveyor is another rod man with another stadia pole (the location of the stadia poles is determined by the survey party chief and is based mainly on topography and the ability to see both poles from where the survey level is set up).  The surveyor looks though the level and calls out the elevation mark he sees on the first stadia rod.  He then reverses direction and calls out the elevation he views on the second stadia rod.  The difference in numeric values he views on the two stadia poles is the difference in elevation between them:

Click on the image to open full size

Additionally, if this is a simple differential leveling job (and I think it is based on the type of level being used), there is another crew of chain men measuring the distance between the two stadia rods.  In 1927 this would have been done using steel survey tapes or chains.

Leveling is slow, tedious and physically demanding work.  There were no old farts out working on leveling parties except perhaps as party chiefs.  The rod men and the chain men were constantly moving, carrying the survey forward.  The instrument man and the recorder were responsible for moving and setting up the level in a new location, and the party chief was moving between all members of the survey party and scouting ahead for new setup locations.

Their work was absolutely critical, though.  The meticulous work of the surveyors of the US Coast & Geodetic Survey and the US Geological Survey created the accurate spatial framework that this countries maps and charts continue to be built upon.

But that's not the end of this story!  I got interested in this picture for a particular reason.  The vertical survey control for the airport I work at was established by this particular USC&GS survey project.  I would like to think that it was these three unnamed gentlemen who, sometime in 1927 or 28, ran their traverse down into College Park, GA and set the single elevation benchmark that became the origin point for all vertical survey work done at the airport until the advent of GPS-based survey in the late 1990s. 

Brian