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I was looking at the below map and got to wondering how much of all of those minute squiggling contour lines are real detail and how many are just artistic flair:

Topographical map of the area west of San Jose

I could imagine that satellite laser/radar imaging could generate this image computationally today, but this map was printed in 1956, back when mapmakers were still etching plates by hand.

Did people actually scour the entire United States measuring elevation at this insanely fine level of precision to produce these maps? To what extent are the contour lines inferred from coarse-grained (maybe aerial) observations like "there's a ridge over there, and it's kind of jagged, so we'll make it squiggle a few times" versus "there are exactly nine squiggles between these two creeks 100 meters apart?"

  • ive always wondered the same thing. I think it was a combination of aerial photography and ground truthing/surveying. Any way they did it could not have been super accurate. – atxgis Apr 26 '18 at 22:23
  • I'm not sure what all the downvotes are for. Just trying to get some insight into the science of "manual" cartography which I know almost nothing about. – Brian Gordon Apr 27 '18 at 2:11
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    stackoverflow.blog/2018/04/26/… – atxgis Apr 27 '18 at 14:20
  • @BrianGordon I didn't downvote, but I can see why someone might. A simple google search turned up: pubs.usgs.gov/gip/topomapping/topo.html , and as you can see, your answerer is just doing internet research and reporting it for you--something that you could do yourself. – Jon Apr 27 '18 at 18:12
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    @Jon The page that you linked there is a good overview, but one might assume from its wording that the OP's map complies with National Map Accuracy Standards, which would give OP a good answer to his question of contour accuracy. However, if you look at the 1956 USGS maps of that scale for that location you will find that they OMIT the statement "This map complies with the national standard map accuracy requirements." ....I assume that it is intentionally omitted, which is why I couldn't specifically answer OP's question about the accuracy of the contour lines on this particular map. – cmrRose Apr 27 '18 at 19:15
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That image looks like a 1:250,000-scale USGS topographic map. Yes, USGS surveyors did in fact "scour the entire United States measuring elevation at this insanely fine level of detail to produce these maps." This particular area was surveyed at this scale by the USGS as early as 1895 and 1899, resulting in the 1902 Santa Cruz Topographic sheet. You can view and download all the historic USGS maps of this area here: https://ngmdb.usgs.gov/topoview/viewer/#9/37.4585/-122.5264\

The first more-detailed USGS maps of this area, at a scale of 1:24,000, all have a publication date of 1955 and state "Topography from aerial photographs by multiplex methods | Aerial photographs taken 1953. Field check 1955" So they did use photogrammetry, as @atxgis said. This document may give insight on the accuracy and process of photogrammetry, which has been in use by the USGS since 1904 (with aerial photography since around 1918, and multiplex methods since 1935): https://pubs.usgs.gov/circ/1952/0218-1952/report.pdf

Here is more information about the USGS Topographic Branch, including several photos of the tools of the trade: https://pubs.usgs.gov/circ/1341/pdf/circ_1341.pdf

This 1956 map would have been compiled from the 1955 maps at 1:24,000 scale as well as other sources, as briefly noted at the bottom of the map sheet in the sources (lower left corner) and Reliability Diagram (lower right area), which probably look like this:

map sources

Reliability diagram

Maybe this information will help you further investigate how accurate the linework might be, since the linework in your map specifically was based on "Topography from aerial photographs by multiplex methods."

Sorry for all the edits, but you got me interested in digging deeper. I found this video showing the process of topographic map production using aerial photography: "Topographic Mapping by Photogrammetric Methods (1947)" https://youtu.be/CGTlOOpjTRc

  • @BrianGordon I don't know how the 1:250,000-scale maps were compiled from the many different sources, and I don't know how the contours were generalized (i.e., smoothed/reshaped to be readable at this scale) in the process. – cmrRose Apr 27 '18 at 22:16
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On the map on which you look there is no real detail, because the cartographers created it by the methods of photogrammetry and cartographic generalization. Creating a map in the past was an art, but automating in the near future will change that false perception. Maps in the past are beautiful pictures, automation will work with real objects and visualize those objects of terrain that you are interested in, with the scale (with the accuracy) that you need.

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