While looking for good topography maps for hiking I came across Topozone.com which truly amazed me with its ability to display the topo map of anywhere in the united states right down to 1:25,000 which is detailed enough to hike just about anywhere by.
My question is how did the USGS manage to record the topography of every square mile in the United States? The task seems monumental when you consider the sheer size of the nation and the fine detail that these maps convey. Did someone actually sit there and draw all those billions of curving lines with such accuracy?
Collecting the data was probably the more difficult undertaking. I’m sure somebody who knows something about pre-computer age surveying will be along to comment.
As for sketching contour lines by hand, and dealing with the data …
When I was in grad school, our department was doing a number of projects involving interactive display of topo maps on Tektronix storage tubes backed by an HP 2100 - things like figuring out where to drag logs out of the woods in logging operations, and so on.
Now, where to get the data - well, the Civil Engineering department had a digitizer - basically a big huge table you could run around on with something like a mouse, clicking on coordinates, and they paid students $3 / hr to digitize topo maps, clicking around contour by contour.
The rub was that the digitizer was hooked up to some discrete TTL toy machine that had a teletype connected to it, and could run a few hundred line BASIC program without running out of memory. CE was adamant that their digitizer and computer was NOT going to be hooked up to anything else, either. The ONLY way to transfer the data was on paper tape reels from the teletype’s tape punch.
The data for our fancy graphics labs was thus very unpleasantly obtained by dealing with many, many HUGE reels of paper tape which had to be read on the campus mainframe, massivly massaged and reentered to fix the innumerable errors in data entry, and transferred again to the HP.
When I was in surveying school about 20 years ago most topographic maps were made by photogrammetry. This entails some preliminary ground survey work to provide set reference points visible from the air. Aerial photographs would then be taken and converted into topographic maps through spectrographic analysis.
At that time contours were still traced manually with a spectroscope by moving a floating dot around the 3d image. At that time the output was justbecoming digitized but I imagine almost the whole process is computerized by now.
Photogrammetry is still being used! Field surveys to tie out aerial targets begin the project, and then the flight lines are given to the pilot. The planes have stereographic cameras which photograph 9"x9"contact prints. Using fancy-pants math, the data are constructed to give some VERY impressive final results. I have found that field terrain surveys of hard surfaces show photogrammetrically acquired terrain results within a half foot.
The key is in the stereographic photography. You can place the contact prints side-by-side, and then using special stereo lenses, actually see the terrain features “pop up.”
~VOW
Except that you didn’t say how the elevation data was taken, which is sort of the whole point of the question. I, for one, didn’t realize from your description that stereo photographs were used.