Height Technical Information

Transcription

1 Height Technical Information Blackwell s has compiled the most comprehensive range of height information products in terms of detail and coverage from one source. If you combine height data with detailed MasterMap and/or Aerial Imagery you can create a detailed 3D image of your area. This will help give a full understanding of the area and it s surrounds in a completely unique way. The two different versions that all height data is available as: Digital Surface Models (DSM) DSMs measure the height values of the first surface on the ground as seen from above. This includes terrain features, buildings, vegetation and power lines etc. They therefore provides a topographic model of the earth's surface Digital Terrain Models (DTM) DTMs provide a topographic model of the bare earth / underlying terrain of the earth's surface. They are usually derived from DSMs by digitally removing the cultural (man-made) and vegetation features described above. Which version do I need - Terrain (DTM) or Surface (DSM) Models? As a result of the above processes, the stated accuracy of a DTM will usually be slightly lower than that of the DSM from which it is derived, particularly in areas of continuous and dense building and tree coverage. Formats: There are 6 main formats of height data offered on All require CAD or GIS software to open and use the product: DXF Format Points CAD (.dxf). ASCII Grid for GIS (.asc). DXF 3D Face CAD (.dxf). XYZ easting, northing, height CAD/GIS (.xyz). Erdas Imagine Image file GIS (.img). Contours DXF files for CAD/GIS (.dxf). Most common used are: Contours DXF files for CAD/GIS or DXF format points (.dxf). In most cases an Ordnance Survey base map or aerial imagery will be needed in order to relate the height information to geographic and topographical features. All formats are geo-referenced with an x, y (eastings, northings on the Ordnance Survey National Grid) location when opened in CAD/GIS; this means that it is very easy to exactly overlay height data with OS mapping products in CAD/GIS. In CAD an insertion point of 0,0 will need to specified if prompted by the software.

2 We can also supply various other formats of height data on request, although only offline. Explanation of formats: 1) DXF Format Points are a grid of points at evenly spaced intervals based on the resolution of the height data. Each point has a value for x (eastings), y (northings) and x (the height value). Individual Labels for each point s height value are not included within the data (this would greatly increase the file size), however once in CAD the Properties command can be used to display each point s elevation value. Fig.1 Format points when opened in CAD. Fig.2 DXF Format points with OS Mastermap overlay

3 Fig.3 DXF Format points as viewed with the CAD Orbit function, this displays the 3D properties of the data format. 2) ASCII Grid is a squared grid of points arranged in rows and columns; each grid point is geographically referenced and has a elevation value. ASCII Grid can only be used in GIS software; software reads the file in much the same way as an image file, except that instead of a colour for each square, there is a height value. Because the file is only storing a height value based on a pre- defined square grid, ASCII grid has the advantage of having relatively small file sizes. Fig.4 ASCII Grid DSM file shown in Global Mapper GIS, with 3D window.

4 Fig.5 ASCII Grid DTM file shown in Global Mapper GIS, with path profile line of sight. Fig.6 An ASCII Grid DSM file displayed in Global Mapper s 3D function with an aerial image overlay.

5 3) DXF 3D Face is a CAD wire framed surface model which shows a triangle between every height point. This format is also known as a TIN (Triangulated Irregular Network) model, however it is not optimised as some TIN models are. An optimised TIN model will show different size triangles for changes in elevation (larger triangles for flat expanses), however the 3D Face s triangles are all the same size giving quite large file sizes. Fig.7 3D Face DTM viewed with the Orbit tool in CAD. Fig.8 3D Face DTM, viewed with the Render tool in CAD.

6 4) XYZ format is a file containing comma (or other delimiters such as a space or tab) separated values, in this case: easting, northing, height. This can be read by GIS and some CAD software either as points or an elevation grid. XYZ text can be viewed in Microsoft Excel and Notepad. Fig.9 An xyz. File viewed in Notepad showing: easting, northing, height. Fig.10 An.xyz DSM file viewed as an elevation grid in Global Mapper GIS.

7 5) An Erdas Imagine Image file is an image file used in Erdas Imagine GIS, remote sensing and graphics editing software. It is essentially very similar to the ASCII Grid file except that null elevation values do not incur as much file space as in an ASCII Grid file, giving an even smaller file size than the ASCII Grid. 6) Contours in DXF format for use in CAD/GIS are the most widely used and understood way of visualising height information. Contours are only used for representing DTMs and are lines draw between points of equal elevation. The accuracy of the contours depends on the accuracy and density of the height information that the contours are derived from. We provide all the height data with 1m contour intervals, no matter what the resolution, this ensures that contours are present in all areas but doesn t indicate the accuracy of the data. Fig.11 DXF Contours viewed in CAD. Fig.12 DTM Contours in CAD overlayed on an OS Mastermap base map.

8 Fig.13 DTM Contours overlayed on OS Mastermap and an aerial image.