Hunting the edges. Ideally break lines should be digitised off the surface mesh but for speed these lines are traced of the ortho-photo as polylines then matched to the required z value before reducing them to plain 2d lines and adding them to the model. The near flat profile of the features mean that break-lines are very disruptive to the contour model.
The main value of the break-lines is the re-enforcement of the surface under the hedge lines, longer lines being more disruptive than shorter ones the lines were trimmed after trial contour generation.
On the left before break-line on the right after. The break-line along the edge of the central square serves to reconstruct the pattern on the Northern and Eastern embankments. The impact was not aggressive in sharpening the shape in areas with few points (under vegetation) but some increase in edge definition has occurred but also disruption of the ‘pocket’ shapes in the SW and SE bastions; the break-line was trimmed out of the bastions and re-contoured. To minimise model disruption the break-lines are short segments of much longer form lines.
Detecting the points in error and fitting form lines along the hedge lines requires close monitoring of Z values the XY component is verified from the image. TheoContour manages break-lines by layer so checking and editing them is easy, the layer is renamed to turn them on and off. It’s a graphical process, matching the contour flow to the form shown on the ortho image.
Ortho-image with contour overlay. An overlay to the ortho image provides a record, not only of the surface vegetation, but also the relative heights of the historic features. The contour plan clearly shows the surface denudation in the SE corner of the monument, a result the of recent winter flooding of the washes.
Some experiment in presentation colours is needed:
Next: smoothing, fixing the datum, adding features, text and line weighting.