171-5 How accurate was paper geologic mapping?: A quantitative assessment

Session: Advancing Geologic Analysis with Digital Outcrops and Close-Range Remote Sensing Data

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Abstract:

We evaluate the accuracy of paper geologic mapping using a structure-from-motion (SM) terrain model with submeter resolution as reference.  Our paper map was made in 1980s using only a  1:63,360 topographic base map with 100 ft contours enlarged to 1:24k for fieldwork and no georeferencing during field work beyond map inspection and resection.  Topographic relief is large (~ 1km) with extensive areas of slopes >60^o.  We also evaluate a published compilation of that map to 1:63,360 with modifications by the compiler.  Our reference SM model was obtained from two fixed-wing, airborne surveys in 2021:  a vertical imaging survey and an oblique imaging survey.  The SM model, for both surveys, have resolutions at the dm to cm level.    We quantify difference between older data and the reference with four metrics:  1) average vector difference, 2) average 2D distance,  3) average 3D distance, and 4) average distance from bias corrected curve (mean removal) we refer to as scatter.  All use vector differences computed from the horizonal normal to the reference curve.  Some initial results include:  1) visual inspection shows an obvious difference where increased resolution of SM data produces line curvature at smaller scales, shown by the scatter metric which is consistently higher with anything but near vertical structures. 2) high-angle contacts were generally relatively accurate (typically within 20m of model reference).  Exceptions can be attributed to mis-location from insufficient contours; 3) moderately dipping contacts were generally relatively well located (errors <20m) where topographic contours were reasonably detailed  but, not surprisingly, contacts on dip slopes and uniform slopes were locally mislocated as much as 100m;  4)  highly irregular, intrusive contacts exposed on steep slopes show low mapping accuracy, due largely to generalizations of the irregular contact on the paper map; 5) worst were the compilations of the field maps to a 1:63,360 published map where many map details at 24k were simplified (e.g. law-of-v’s details removed) or remapped to produce larger errors than the original mapping (local errors >150m with high average errors due to poor rendering of contact curvature). 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8357

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