20240617 10225000 1.0 ISO19139 150000000 5000 20240626 11401894 editor:esri.dijit.metadata.editor ISO 19139 Metadata Implementation Specification false TRUE <div style='text-align:Left;'><div><div><div style='font-size:12pt'><p><span><span>This dataset shows the contours mapped in 2002 over the Brisbane City Council Local Government Area (LGA). The 2002 contour dataset uses the Geocentric Datum of Australia 1994 (GDA94) datum and is projected in Zone 56 of the Map Grid of Australia (MGA56).</span></span></p></div></div></div></div> Contours — 2002 2024-06-24 2024-06-24 2024-06-24 0.0 0.0 0.0 0.0 1 opendata@brisbane.qld.gov.au +61 7 3403 8888 brisbaneopendata Open Data property and planningspatial datacontourmaparcgiselevationtopographycartography Contours mapped in 2002 over the Brisbane City Council Local Government Area © Brisbane City Council 2024 © AAM Surveys Pty Ltd 2002 <DIV STYLE="text-align:Left;"><DIV><DIV><DIV STYLE="font-size:12pt"><P><SPAN><SPAN>This material is licensed under a </SPAN></SPAN><A href="https://creativecommons.org/licenses/by/4.0/" STYLE="text-decoration:underline;"><SPAN STYLE="text-decoration:underline;"><SPAN>Creative Commons - Attribution 4.0</SPAN></SPAN></A><SPAN><SPAN> licence.</SPAN></SPAN></P></DIV></DIV></DIV></DIV> Other Constraints Other Constraints Background In June 2002, Brisbane City Council commissioned AAM GeoScan to provide a Digital Elevation Model (DEM) and a contour dataset covering the Brisbane City Council local government area. The contour and digital elevation model data is designed to: support council’s planning, investigation and preliminary design activities assist compliance monitoring provide a basis for flood modelling in various creek catchments support production of hardcopy maps and digital map products for sale to the public. Solution AAM GeoScan defined the total Brisbane LGA area using Airborne Laser Scanning (ALS). This is a survey technology whereby a distance-measuring laser is carried in an aircraft and measures millions of spot elevations on the surface below. The laser will reflect from (and so measure) both ground and above-ground points, including buildings, vegetation and even powerlines. From this wealth of discrete elevations, the datasets required by BCC were compiled. Collected Data An estimated 664 million data points were collected across the project area, with an average of 1.3 metres between the points. Accuracy In the Flood Plains and Residential Areas, 90% of all well-defined spot elevations and DEM points are within 0.15m of their true elevation and the remaining 10% do not exceed 0.3 metres of their true elevation. In the Non-Residential Area (Zone 2 in Figure 1), 90% of all well-defined spot elevations and DEM points are within 0.3 metres of their true elevation and the remaining 10% do not exceed 0.6 metres of their true elevation. Classification Because the laser reflects from both ground and above-ground features, a series of routines are applied to classify the data into “ground” and “non-ground” laser strikes. This automatic classification is based on terrain shapes by iteratively building a triangulated surface model. The routine starts by selecting some local low points as sure hits on the ground. The routine then moulds the model by iteratively adding new laser points to it. Each added point makes the model follow ground surface more closely. Iteration parameters determine how close a point must be to a triangle plane. This determines whether the point can be included in the terrain model. After the automatic classification, AAM’s operators compared the ALS data against BCC-supplied orthophotos and corrected any areas where the automatic classification had failed. Areas notorious for requiring manual classification include: 1. shopping centres - where the software fails to detect that a large flat building is not flat ground. 2. overpasses - a road is obviously “ground” and a bridge “non-ground”, but the transition from one to the other is less well defined. 3. water courses - whilst the laser is largely absorbed by rivers and lakes, the few returns one does get should not be classified as “ground”. Minor classification anomalies will remain in the data even after manual comparison with orthophotos. Ground Definition As the measurement source is from a low-flying aircraft, the ground surface will be very well defined in clear and open terrain. In areas of dense vegetation, the percentage of laser points which penetrate the canopy and define the terrain is proportional to the density of the canopy. In areas where only dappled sunlight is striking the ground, the ground will be less-well defined, but ALS offers the best definition of the surface without employing teams of field surveyors to coordinate the ground by total station. Tall buildings and bridges can create shadows of data on the side away from the overflying aircraft. In the Brisbane CBD area, this effect was minimised by acquiring data in both east-west and north-south flight lines. Contours The project required 0.5m contours to be provided in the Flood Plains and Residential Areas, and 1m contours in the Non-Residential Areas of Zone 2. The ALS ground point data was triangulated and a set of raw contours derived for each BCC 1:5000 sheet. Where 0.5m contours would not fit within the Microstation .dgn file, the contour interval was reduced to an interval that would. Because the DTM software honours the wealth of closely spaced points, raw contours derived from ALS ground points are technically correct but notoriously ugly. The ALS ground points were therefore resampled using a nearest neighbour approach to find a best fit plane equation from localised points at similar elevations. This process creates a smoother terrain model from ALS data and produces more cartographically acceptable contours. Manual editing further improved the cartographic appearance of the contours. This editing involved moving or deleting contours where data shadows affected the contours’ true position. Such editing is typically required under bridges and along vegetated embankments. In areas where the number of ALS ground strikes fell to less than 50% of the total ALS strikes, contours were reclassified to “approximate contour”. This approach was found to be inappropriate in heavily built-up areas. AAM therefore revisited the tiles in inner-Brisbane and used a more manual process (with the orthophotos) to decide what is an approximate contour and what is not. Both the raw and cartographic contours were supplied in BCC’s DGN seed file format. 1719365541271r9989030572023581 2024-06-26 11:30:00.00