GSD-Elevation data, Grid 2+

Transcription

1 Date: Document version: Product description: GSD-Elevation data, Grid 2+

2 LANTMÄTERIET (12) List of contents 1 General description Contents Geographic coverage Delivery tiles Reference system Quality description Metadata Creating a grid Contents of the delivery Folder structure at delivery Delivery format Delivery of coordinate-transformed grid Set of files and contents Description of image file in metadata - Point density of laser points classified as ground Description of the content in the XML file Appendix A The production and scanning areas

3 LANTMÄTERIET (12) 1 General description Having been commissioned by the Swedish government, Lantmäteriet has been working to produce a National Elevation Model with high accuracy since Elevation data is collected using aerial laser scanning and then processed. Based on the laser points which are classified as ground points, a terrain model in the form of a grid, also known as a raster, is produced. 1.1 Contents The product Grid2+ consists of a terrain model in grid form with a resolution of 2 metres. The product is delivered with metadata, which details origin and processing history. 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 (12) 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 (can also be transformed and delivered in any regional SWEREF zone). 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 4 below.

5 LANTMÄTERIET (12) 3 Creating a grid A comprehensive terrain model in the form of a grid is created from laser points classified as ground and water. The calculation is done through linear interpolation in a TIN (Triangulated Irregular Network). This method conserves the terrain forms very well, but it is sensitive to single incorrect points that may have a great impact on the result. Synthetic laser points have also been used when creating terrain models using laser data with classification level 2 and higher (see Section 4.2 in Quality description of National Elevation Model). At many bridges, the point cloud is manually supplemented with points along the shoreline, or in some cases the underlying roadway, in order to achieve a good interpolation in the TIN. This is due to the fact that points are normally missing on the water surface and on the shore underneath the bridge. These synthetic points are not included in deliveries of laser data and cannot be ordered separately. Starting in autumn 2014, newly mapped shorelines - the shoreline that applied during the scan - are being gradually introduced as breaklines when producing the model (classification level 3, see Section 4.2 in Quality description of National Elevation Model). This is being done for watercourses wider than 6 metres and lakes larger than 0.25 km², which means that these bodies of water have smooth and welldefined surfaces with a single height value (lakes) or sloping surface (watercourses) in the terrain model. The water polygons are not included in the deliveries of the product and cannot be ordered separately. The terrain model being produced has a height value every two metres.

6 LANTMÄTERIET (12) 4 Contents of the delivery 4.1 Folder structure at delivery Below follows an example of a delivered terrain model and metadata from one of the scanning areas.

7 LANTMÄTERIET (12) 4.2 Delivery format GSD-Elevation data, Grid 2+, is provided and supplied in the format of an ASCII Grid. Example of the ASCII grid file's content: GSD-Elevation data, Grid 2+ can also be provided in ASCII tabular format. The delivery will then contain columns separated by tabs, from left: E (Easting), N (Northing) and H (Height). Example of the ASCII tabular file's content: The grid files are supplied compressed in RAR format (software to unzip files is free to download on the Internet).

8 LANTMÄTERIET (12) Content in ASCII Grid: Example of content Explanation ncols 1250 nrows 1250 xllcenter yllcenter cellsize nodata_value Number of values/rows Number of rows E co-ordinate of the grid point (the pixel centre) in the lower left-hand corner of the area.* N co-ordinate of the grid point (the pixel centre) in the lower left-hand corner of the area.* Grid size/resolution Value used when height value is missing. The grid covers water areas as well, so this value should not normally occur. Elevation figures for the rows, given in metres with two decimals. Value is given from west to east and from north to south. See example below. For file content, see above. *The file header changes slightly during the transformation of an ASCII Grid to a system other than 99 TM. The insertion point for the south-western pixel's south-west corner is described xllcorner and yllcorner. Example: osv. Rad 1 osv. Rad 2 xllcenter yllcenter The figure shows the position of the grid point in relation to an imaginary pixel with a two metre side.

9 LANTMÄTERIET (12) 4.3 Delivery of coordinate-transformed grid Grid and tabular data is normally delivered in SWEREF 99 TM, but can also be transformed and delivered in regional SWEREF-projection zones. However, the metadata included in the delivery will still be adapted to the tiles in SWEREF 99 TM. Transformed grid and table is supplied uncompressed. The transformed tiles are given a new name according to the projection's index classification. To reproject elevation data in ASCII grid-format from SWEREF 99 TM to a regional SWEREF-zone the data will go through both a rotation of the raster and a resampling of the raster cells. An interpolation of the height values is necessary in order to adjust the elevation data to the regional SWEREF-zone grid. The method used is a bicubical interpolation method, as this method takes the surrounding terrain into account. A test made at Lantmäteriet during 2012 showed that the loss of quality in the resulting reprojected grid was negligible. Reprojection of gridded elevation data in tabular-format is a 2D-reprojection, which means that the height values in the data is untouched. 4.4 Set of files and contents File name (example) 67475_5875_25.asc (supplied compressed in.rar) 09P001_67475_5875_25_density.tif 09P001_67475_5875_25_density.tfw 09P001_grid_21850.xml (scanningarea_grid_id-nr.xml) kml (ID-nr.kml) Description The file name comprises the co-ordinates of the lower lefthand corner of the tile, the size of the tile given in 100s of metres, and the file format. A raster file presenting the point density for those laser points that have been classified as ground. See description Geo-referencing file for the image file above. Metadata presenting origin and level of processing. (See Section 4.4.2). File showing the area's extent in Google Earth, in SWEREF 99 TM.

10 LANTMÄTERIET (12) Description of image file in metadata - Point density of laser points classified as ground Every delivered grid file is accompanied by an image illustrating the degree of detail with which the terrain model can be expected to represent the ground surface (the file is named, for example, 09P001_67475_5875_25_density.tif). The image has a resolution of 10 metres and shows the average point density of ground in laser data. The point density is illustrated using colours as per the table below. Each file covers a 2.5 x 2.5 kilometre processing tile. Colour Point density Comment Blue > 0.5 points/m 2 On open areas and on side overlaps there might be more points on ground than the specified minimum requirement of 0.5 points/m 2. Green points/m 2 Yellow points/m 2 Red > points/m 2 On average there is at least one point on ground within one grid cell (corresponding to 2 2 metres). On average there is at least one point on ground within four grid cells (corresponding to 4 x 4 metres). The terrain model may have a diminished degree of detail in these areas. On average there is less than one point on ground within four grid cells (corresponding to 4 x 4 metres). This might be caused by dense forest, steep slopes 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, buildings with black roofs or new asphalt. Example: The colours in the density image above represent varying point densities on ground in the laser data.

11 LANTMÄTERIET (12) Description of the content in the XML file The following is an example of content in the metadata file scanningarea_grid_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 check point surfaces used to control laser data 7 Number of ground checkpoint surfaces used to control laser data 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. This is followed by information on each supplied tile included in the scanning area: Field Example Explanation Ruta (Tile ID) Skanningsdatum (Scanning date) Klassificeringsprogramvara (Classification software) 67475_5875_ , TerraScan Co-ordinates of the lower left corner of the tile, and coverage on the ground given in 100s of metres. Date of the original scan. One tile normally contains points from more than one flight line, and these may have different dates. The software and version most recently used to classify the ground points that form the

12 LANTMÄTERIET (12) Field Example Explanation basis of the grid. Klassificeringsnivå (Classification level) Senaste klassificeringsdatum (Last classification date) 1 Scope of classification. See Quality description of National Elevation Model Date of the most recent classification. Gridprogramvara (Grid software) TerraScan Software and version used to create the grid. Griddatum (Grid date) Ajourföringsanledning* (Update reason) Ajourföringsmetod* (Update method) Date the grid was created. 0 0 None 1 New measuring 2 Changed classification 3 Quality improvement 4 Updating 5 Densification 6 Correction of data 0 0 No update has been made 1 Airborne laser scanning 2 Photogrammetry 2,500 metres 3 Photogrammetry 4,800 metres 4 Geodetic surveying (terrestrial) 5 Airborne radar 6 Digitisation 7 Adjustment Ajourföringsomfattning* (Update scope) *Ambition level for updates has not been determined. 0 0 Original data collection 1 Complete update (the whole tile is controlled and updated) 2 Individual features (individual points or lines has been added or deleted)