In case it's easy to forget what a tremendous advance in mapping accuracy took place in the 20th century in the US, here is further evidence. This image is the Texas Highway Department map of Hood and Somervell Counties from 1958 (the darker map) superimposed upon the map of the same area from 1936 (the lighter blue map), also prepared by the Highway Department.
Note that the graticules of latitudes and longitudes on both maps are essentially alligned, both nominally NAD27. I'll also mention that the channel of the Brazos River in the area shown hasn't significantly changed for at least the last century, so the wild differences in its position are simply mapping errors.
> This image is the Texas Highway Department map of Hood and Somervell Counties from 1958 (the darker map) superimposed upon the map of the same area from 1936 (the lighter blue map), also prepared by the Highway Department.
The main problem, however, is that the 1928-vintage plane table mapping agrees much more closely with the later 1958 photogrammetric mapping, which raises the obvious question of how the DOT mapping in 1936 could have drifted so far into Fantasia. I suppose it's an interesting question from an academic standpoint, but on the practical level, why does any DOT do anything unless a memo has been distributed?
The 1936 map looks like it might be a very poor combination of bad ground locations of the roads with an overlay of uncontrolled/adjusted aerial photography. After World War 2, some pretty impressive gains had been made in world of aerial photography.
A very nifty comparison.
Thanks.
:good:
Is the 1958 mapping actually more accurate? In all mapped areas?
I got my feet wet in Surveying by being trained as a stereo plotter operator... it is rather easy to do great mapping for the prime area of interest but have a second (novice) drafter fill in the blanks with the "etch a sketch" (that would be me). Luckily Cousin Dick was very anal on QA/QC and none of my work saw the light of day 😉
Great examples, Thanks.
PS: is later topo available for a third overlay?
> Is the 1958 mapping actually more accurate? In all mapped areas?
> PS: is later topo available for a third overlay?
Yes, the 1958 mapping is considerably more accurate than the 1936 mapping throughout the whole county. The reason is that the 1958 mapping apparently was compiled from some preliminary manuscript version of the 1961 edition of the USGS 7.5-Minute Quadrangle maps covering the area.
The USGS mapping accuracy was quite good. It's a bit amazing how well it overlays current 0.5m orthophotos.
The 1936 aerial photography does not appear to well controlled; the 1958 mapping is.
> The 1936 aerial photography does not appear to well controlled; the 1958 mapping is.
I doubt very much that the 1936 mapping was compiled from any aerial photography at all. The available USGS 15-minute quad sheets were all mapped by plane table methods and the additional details such as roads were evidently plotted either from right-of-way maps (in the case of the state highways) or from odometer and compass recon methods.
Most of the aerial photography of rural Texas dates from after WWII and is, of course, the major reason for the large improvement in mapping accuracy in the 20th century.
Dittos on the plane table.
