1(11) Date: Document version: Product description: Laser data

Transcription

1 1(11) Date: Document version: Product description: Laser data

2 LANTMÄTERIET (11) List of contents 1 General description Contents Geographic coverage Delivery tiles Reference system Quality description Metadata Contents of the delivery Folder structure at delivery Delivery format Set of files and contents Description of image files in metadata Description of the content in the XML file Appendix A The production and scanning areas

3 LANTMÄTERIET (11) 1 General description Having been commissioned by the Swedish government, Lantmäteriet has been working to produce a National Elevation Model with high accuracy since Laser scanning was chosen as the method for data collection. In the subsequent processing, the laser points have been classified as points representing ground, water and bridges. Other points have been left unclassified. 1.1 Contents The product Laser data consists of a point cloud in which each point is classified as ground, water, bridge or unclassified (which also includes any incorrect points). The product is delivered with metadata that indicates the origin and status of the processing performed, in addition to a full set of metadata from the scan, including flight lines. If required by the user, files with overlapping data, "border files", can be ordered. These files contain data in a zone that overlaps the neighbouring scanning area by approximately 200 metres. If these files are desired, this must be specified when placing an order. The content of each dataset is presented in Section 3.3 Set of files and contents. 1.2 Geographic coverage Using laser data as a basis, a bare earth terrain model (an elevation model) is being created that covers all of Sweden. The progress of the production is presented on Lantmäteriet's website, see under Höjddata (only available in Swedish). Leverantörens veckorapport (The provider s weekly report) A file which shows more detailed information about the scan, the time of the different steps, the equipment used and the number of flight lines. Produktionsstatus och klart i lager (Status of scanning and Ready for delivery) Presented to give users the opportunity to undertake field surveys in close proximity to when scanning is taking place. The overview presents areas at three different status levels; where flight-line planning is approved, where scanning has started and where scanning is provisionally complete (rescanning may be considered if the quality is not approved during the subsequent verification). Once scanning is completed, it may take up to approximately 6 months before the laser data is ready for delivery to end users. Ready for delivery shows those areas that are ready for delivery to end-users, with a classification level for each scanning area. Sweden is divided into production and scanning areas, se Appendix A.

4 LANTMÄTERIET (11) 1.3 Delivery tiles The smallest unit for processing and delivery corresponds to a tile measuring 2.5 x 2.5 km, known as a processing tile, geographically adjusted to the index system in SWEREF 99 TM. The name of the tile is produced from the coordinates of the lower left corner given in hundreds of metres, followed by the side's length in hundreds of metres, e.g _5975_ Reference system In plane: SWEREF 99 TM In height: RH Quality description See the pdf-file Quality description of National Elevation Model, which was supplied within the delivery. The document can also be found alongside this product description at the website Metadata Metadata (imagery and XML) is supplied with the product, showing information about data capture, editing and point density. Read more in section 3 below.

5 LANTMÄTERIET (11) 3 Contents of the delivery 3.1 Folder structure at delivery At delivery the LAS files, including the metadata, is sorted into a folder with the same name as the scanning area to which it belongs in order to keep common metadata together. The example below shows the folder structure. Metadata for the entire scanning area is delivered in a subfolder to the scanning area.

6 LANTMÄTERIET (11) 3.2 Delivery format Laser data is supplied in LAS format, version 1.2, in point data format 1 with intensity and week time. The LAS files, and any border files ordered, are supplied compressed in RAR format (software to unzip these files is free to download on the internet), which means that the file size has been reduced to approx. 1/5 of the original size. 3.3 Set of files and contents File name (example) 09P001_67475_5875_25.las (supplied compressed in.rar) 09P001_67475_5875_25_density.tif 09P001_67475_5875_25_density.tfw 09P001_las_21850.xml (scanningarea_las_id-nr.xml) 09P001_coverage.tif 09P001_coverage.tfw 09P001_density.tif 09P001_density.tfw 09P001_tasq_bild.tif 09P001_tasq_bild.tfw _ trj Skanningenheter.txt kml (ID-nr.kml) 09P001_67475_5875_25.las (supplied compressed in.rar) Description The file name includes the identity of the scanning area, the coordinates of the lower left corner of the tile, the size of the tile given in 100s of metres and the file format. A raster file with the point density for those laser points that have been classified as ground, one for each 2.5 x 2.5 kilometre tile. See description Geo-referencing file for the image file above. Metadata presenting the origin and level of processing (see Section 3.3.2). A raster file presenting the point density of the laser point cloud, last and only return, for the entire scanning area. See description Geo-referencing file for the image file above. A raster file for the entire scanning area presenting the point density of those laser points that have been classified as ground. Geo-referencing file for the image file above. Presents the height deviations in the overlaps between the flight lines within the scanning area. See description Geo-referencing file for the image file above. Trajectory data, one file for each flight line included in current scanning area. The file name may vary depending on the equipment used, but it will always end with the extension.trj. In the file containing metadata for the flight lines, the name of each trajectory file is connected to the corresponding flight line. All scanning units that have been used during scanning of the country. Not only those related to the current delivery. File showing the area's extent in Google Earth, in SWEREF 99 TM. Border files. Contain laser data in a zone that overlaps neighbouring scanning areas by approximately 200 metres. The files have the same name as regular LAS files, but are kept in their own folder, named according to 09P001_kant. PLEASE NOTE! The files must be ordered specially!

7 LANTMÄTERIET (11) Description of image files in metadata Point density - last and only return For each scanning area the point density of the laser point cloud is presented as an image file, with a resolution of 10 metres. The file is named, for example, 09P001_coverage.tif. Only points from the last of several returns or the only return are shown, i.e. points that may represent the ground surface. The point density of the laser point cloud is illustrated with colours as per the table below: Colour Point density Comment Blue > 1 point/m 2 Green > 0.5 points/m 2 Yellow > 0.25 points/m 2 Red < 0.25 points/m 2 Black 0 points/m 2 Black colour in the image is either due to the water surfaces having been removed by masking or holes in the laser point cloud resulting from poor reflection. Seas and areas outside the national boundary that have not been scanned are also presented as black. The image above shows point density in the laser point cloud, the last and only return, for all of the scanning area.

8 LANTMÄTERIET (11) Point density laser points classified as ground An image with a resolution of 10 metres shows the average point density of laser points that have been classified as ground. Each file covers a 2.5 x 2.5 kilometre processing tile. The point density is illustrated using colours as per the table below. The file is named, for example, 09P001_67475_5875_25_density.tif. The corresponding raster image can also be delivered for the whole scanning area. Colour Point density Comment Blue > 0.5 points/m 2 On open areas and on overlaps between flight lines there might be more points on ground than the minimum requirement of 0.5 points/m 2. Green points/m 2 On average there is at least one point on ground within one area of 2 2 metres. Yellow points/m 2 On average there is at least one point on ground within an area of 4 x 4 metres. The terrain model may have a diminished degree of detail in these areas. Red > points/m 2 On average there is less than one point on ground within an area of 4 x 4 metres. This might be caused by dense forest or water. The terrain model may have a significantly diminished degree of detail in these areas. Black 0 points/m 2 Black colour in the image is due to either the water surfaces having been removed by masking or holes in the laser point cloud. Holes in the laser point cloud are due to poor reflection or dense vegetation, which may cause a total loss of points on ground. Poor reflection occurs on, for example, water surfaces and newly laid asphalt. Example: The colours in the density image above represent varying point densities on ground in the laser data.

9 LANTMÄTERIET (11) Elevation accuracy in side overlaps Statistics showing height deviations between flight lines within each scanning area are presented in an image file with a resolution of 10 metres. The file is named, for example, 09P001_tasq_bild.tif. Height deviation between flight lines. Blue colour represents a deviation of <0.10 metres, green metres, yellow metres and red >0.50 metres between flight lines.

10 LANTMÄTERIET (11) Description of the content in the XML file The following is an example of content in the metadata file scanningarea_las_idnr.xml. The XML schema is available here: The first part of the file provides information common to the entire scanning area. Field Example Explanation Områdesnamn (Scan area) 09P001 Identity of the scanning area. Ursprung (Origin) Höjdnoggrannhet (Height RMS) Kontrollytor i höjd (Checkpoint surfaces in elevation) Kontrollytor i plan (Checkpoint surfaces in plane) 1 1 = Lantmäteriet's laser scanning m The result of the check against known points. The points lie on open, flat, paved surfaces. 9 Number of ground checkpoint surfaces in elevation. 7 Number of ground checkpoint surfaces in plane. Kvalitetsanmärkning (Quality note) Klassificeringsprogramvara (Classification software) Large amount of low vegetation incorrectly classified as ground points. TerraScan Indicates that deviations or problems of which the user should be aware have been detected during processing. The software used to classify all tiles within the scanning area at classification level 1. Other software or other versions may have been used for individual tiles with a higher classification level. Ursprungligt klassificeringsdatum Date of the original classification. (Original classification date) This is followed by information on each supplied tile included in the scanning area: Field Example Explanation Ruta (Tile ID) Skanningsdatum (Scanning date) 67475_5875_ , Co-ordinates of the lower left corner of the tile, and coverage on the ground given in 100s of metres. Date the tile was scanned. One tile normally contains points from more than one flight line. These flight lines will occasionally have different dates.

11 LANTMÄTERIET (11) Klassificeringsprogramvara (Classification software) TerraScan Software and version most recently used to classify the laser points. May relate to automatic or manual classification. Klassificeringsnivå (Classification level) Senaste klassificeringsdatum (Last classification date) Klassnummer (Class number) Antal punkter (Number of points) Totalt antal punkter (Total points) Lägsta höjd (Lowest elevation) Medelhöjd (Mean elevation) Högsta höjd (Highest elevation) Antal enda ekon (Return only) Antal första ekon (Return first) Antal mellanekon (Return intermediate) Antal sista ekon (Return last) Lägsta intensitet (Intensity low) Medelintensitet (Intensity mean) Högsta intensitet (Intensity high) 1 Scope of classification. See Quality description of National Elevation Model Date of the most recent classification. 1 Classes that may be included. See Quality description of National Elevation Model Number of points in each class. 12,023,602 The total number of laser points The minimum elevation in the point cloud The mean elevation in the point cloud The maximum elevation in the point cloud. 7,874,089 The number of laser pulses that only gave one return. 1,978,341 The number of returns that were the first of many. 195,842 May be both second or third return, but not the first or last. 1,975,330 The number of returns that were the last of many. 0 Lowest registered intensity. 75 Mean intensity. 255 Highest registered intensity. On Lantmäteriet's website, Höjddata/Fakta-om-laserskanning/Planer-utfall-ochstödmaterial, under Läge och skanningsdatum för stråk, there is a file to connect laser points to the date of the laser scanning.