Mannsverk road-side outcrop and reference data

This property is particularly useful to other users outside the geoscience world because many applications exploit planes. The stereogram dialog box enables normal interactive queries tab Interactive filter. Ticking “Filter facets by orientation” enable setting a dipdip direction value, with specified span to view which facet is concerned with a given orientation. The spatial display will be updated with only the object belonging to the selected subset. Further, when the filter is enabled, the user may click at a given position in the stereogram. This will update the dipdip direction values of the filter. This is very practical when a cluster of normals stands out in the stereogram and the user want to check it out. Each query may produce a subset sample when hitting the “Export”. Selected objects facets or points will create an new object in the database tree. There is one particular application where dip direction span may be set to 360°, it is to isolate objects which may point in any azimuth but for which the dip is vastly different. This is case for point clouds of buildings, underground quarries corridors, etc. Ceilings and floor have a normal pointing up or down, whereas wall have normal pointing horizontally. The stereogram is a very quick way of separating them in one click. 2.5 Facet export as shapefiles and ASCII CSV Fully aware that no single software fulfils everyone’s requirement, CloudCompare is no exception, FACETS was designed to pass on the facets, and planesfamilies to third arty software in the most practical way. Geologists often use Geographical Information Systems for mapping and interpreting their data, shapefiles, was therefore a must to integrate outcrop orthophotos with facets. The shapefile contains the 3D polyline contour as well as all attributes of the facets in the attribute table elementary id, centre, nomal, rms, horizontalvertical extent, surface, dip direction, dip, family index and plane index. CSV export does only contain the attributes of the shapefile without the actual polyline information. This is amenable to direct import in many common structural geology tools. 2.6 Guessing appropriate maximum distance The maximum distance of a point to a best-fitting plane is a criterion which is difficult to appreciate off hand. Which value is best? Well, it depends … on the point cloud quality. We suggest the following procedure to specify values adapted to your dataset. This is what one did back in the days when FACETS did not exist. Inside your 3D point cloud, identify a planar facet which is unambiguous and isolate it from the rest of the point cloud. Compute the best-fitting plane Tools Fit Plane for this subset. The resulting best-fitting plane is a meshed surface. To check for the statistical values of the residuals, compute a cloud-to-mesh difference Tools Distances CloudMesh dist. Statistical statistical distribution properties can be queried with the histogram tool Tools Statistics Compute Stats Param. Active SF. Quantile values of the displayed histogram appear when clicking at any location within the histogram space. Try this procedure with less obvious facets which you want to make sure to extract systematically.

3. CASE STUDY

3.1 Mannsverk road-side outcrop and reference data

FACETS was applied to a 3D point cloud of road section located in Mannsverk, a suburb east of the city Bergen, Norway 5.3683°E; 60.3571°N. The outcrop faces west with a crudely north-south trend. The rock is an “amphibolite, transformed and severely deformed gabbro and green stone with bands of trondhjemite ” NGU, 150.000 geological map. The outcrop is 20m long and 4m high. Figure 1. Mannsverk outcrop, 20m-wide, 3-m-high, North is to the left, the outcrop is facing West. The geological scan line was surveyed slightly above the hand rail, from this perspective. Following common geological practice, a 15-m-long scan line was established at a height of about 1.70m above the ground to systematically measure all planar facets being intercepted by this line. 59 structural measurements were performed reporting approximate linear coordinate of the facet centroid along the line, the strike and dip of the facet, its eye-balled roughness amplitude and approximate surface area Table 1. This is the explicit population , from a field geologist’s judgement, of all the planes expressed in the outcrop at this height. To ensure correct correspondence between field measurements and 3D cloud computation, each facet was labelled with silver-gray duct tape stuck onto the rock face with compassclinometer readings prior to shoot the photographs. Four main families of planar facets appear from compass measurements dip direction use clock-wise rotation down-to- the-right convention: east-pointing steep N075°E60° and shallow N110°E40°; south-east pointing N150°E87°; and south-west point N210°E65°. Linear coordinat e [m] Dip direction [°] Dip [°] Apparent roughness amplitude [cm] Estimated surface area [dm²] 2.7 90 14 0.5 16 3.1 178 76 2.5 12 3.3 86 66 0.5 18 3.6 171 85 2.5 18 3.8 255 35 1 8 3.9 165 85 0.5 2 4.1 160 86 0.5 2 4.2 160 85 0.5 3 4.6 45 62 0.2 2 4.9 120 64 0.2 0.5 5 195 56 0.2 0.5 5.1 130 64 1 1 5.15 204 56 1 1 5.23 220 52 0.5 0.5 This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B5-799-2016 801 5.3 133 68 0.5 1 5.7 214 65 0.2 0.5 5.75 138 88 0.2 0.5 5.85 214 66 0.2 0.25 6.4 80 67 1 1 6.65 140 86 2 1 7.3 75 56 1 9 7.45 160 88 0.2 6 7.5 220 52 1 2 7.6 144 75 1 4 7.85 214 66 0.5 4 7.95 142 89 1 4 8 228 72 0.5 1 8.1 123 35 0.5 2 8.3 218 64 0.2 0.5 8.4 215 67 0.2 0.5 8.7 120 37 1 12 9.9 59 72 2.5 15 10.45 90 72 0.5 1 10.6 220 86 0.2 1 10.9 73 61 0.2 12 11.1 165 80 0.5 1 11.15 124 83 0.3 1 11.4 62 58 0.3 1 11.6 105 35 0.3 6 11.8 55 53 0.2 6 11.9 120 87 1 3 12.3 104 37 0.5 4 12.3 75 64 1 4 12.4 110 41 0.5 8 12.75 104 45 0.5 4 12.75 75 70 1 8 13 204 78 1 16 13.7 100 42 1 40 14.2 62 45 1 10 14.5 119 84 1 2 15.5 109 34 2 20 15.9 74 65 1 100 16.2 120 84 1 1 16.8 75 66 0.5 21 17.3 115 88 2 18 17.6 55 62 0.5 24 17.7 145 86 3 12 18 75 62 0.5 12 18.3 123 87 2 8 Table 1: Structural measurements of Mannsverk outcrop. Origin of linear coordinates is arbitrary. Dip direction convention follows compass clockwise rotation as in Nxxx°E aimed so with the down-to-the-right rule.

3.2 3D point cloud SFM acquisition