Accuracy of Valuation maps
Railroad built in 1911. I have the valuation maps that show the boundaries of the ROW (not parallel to centerline, the boundary follows cut and fills). It is now a rails-to-trail, no tracks left. There was a road underpass to the west, and and old overhead farm crossing to the east, about 4200 feet between them. Although now gone, I did locate the abutments of the bridge to the west before it was removed, which has station and offset shown on the valuation map. The crossing to the east still has abutments intact, located that as well. There is a curve between them. No r-o-w monumentation.
My question is how accurate typically were the bearings and distances on the evaluation maps? I know that the bearings of course depend on what their basis of bearings are, but I could rotate everything to fit between the underpass and the overpass. If I compute the C/L and the R-O-W and extend it between them, what accuracy could I expect? I am thinking 1 foot would not be a stretch.
The property in question is about halfway between them. Before tying in the overpass abutments to the east, I created a trial alignment that looked pretty good with the roadbed, using the station and offset at the west underpass, but it misses C/L by 14 at the overpass once I tied that in. There is a subdivision on the north side and a subdivision on the south side of the RR. Both encroach by about 5 feet into the R-O-W, but their outside boundaries do track the r-o-w course (i.e. parallel to but offset from), although the one on the north side does converge to meet the R-O-W. They were built late 60’s, when the RR was being abandoned. I tend to trust the RR evidence more than the subdivisions. I fitted the south subdivision by locating the roads, and the north subdivision by the roads and also by four monuments found and tied in. There are monuments in the south subdivision shown on the plat, but they were not found and I believe they were never set, which is typical for this area.
Red is railroad, green is subdivision in the image below. Since both encroach about equal amount, I didn’t want to just rotate the RR to fit the evidence at the abutments. Of course, I realize that the abutments in the east may not have been symmetric about centerline, as there is no offsets shown for that structure. I am trying to get more maps that might show how they were constructed.
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This discussion was modified 1 month, 2 weeks ago by
john-hamilton.
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This discussion was modified 1 month, 2 weeks ago by
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I’ve usually seen about 1-2 minutes for the curve data (tangent-tangent) lines. If they say a 20d15′ delta I can expect a delta that’s close to the record. Also RR were done true north for the most part, along east-west lines expect curvature to come into effect. RR were often very tightly surveyed. Also be wary of spirals included with curve data. That was often for track and not ROW.
Stationing was measured on the slope.
Maybe get lucky and find some old markers and other pipes structures stunning in called out on the Val maps they are usually called out by stationing and offset as well either by cl or row. I was able to piece together a line that was no longer in by finding some old head wall and marker on one spur line. Degree of curve for railroad once I used that I hit those pretty well within reason. Alignment came in better than expected. Remember they laid tracks in sections the train itself helped to place it in the curve. Those mile marker post sometimes left or broken off at the ground and still in. I am no expert at all. But that’s how I wiggled one in. And to give a sanity check on the math from physical evidence that was on the Val maps.
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