Image Draping & navigation within Virtual GIS

Transcription

1 Image Draping & navigation within Virtual GIS Draping of Geo Corrected data such as aerial imagery or map data enables virtual 3D field tours to be conducted in an area of interest. This document covers the draping of a Geo Corrected image over an existing Digital Elevation Model (DEM) and navigating through the composite model

2 Image Draping This guide assumes the existence of, and access to, a digital geo referenced map slip (see Geo Correcting Digital Images.pdf) and a DEM of the same area Both using the same map projection scheme See guide covering image geo referencing The procedure described covers: Displaying the map slip in 3D Checking the accuracy of the projection Extracting spatial data

3 Image Draping Image draping is conducted in the Virtual GIS viewer (a drop-down menu from the main ERDAS toolbar)

4 Image Draping First, select a DEM from the collection that covers the same area as your field slip (25m accurate Landsat images are currently archived, but more accurate ones may become available in the future) Open this as a DEM within the GIS viewer File Open DEM

5 Image Draping Secondly, open the Geo Corrected map slip as a raster image File Open Raster Layer If more than one image is to be open make sure that the background transparent box is ticked in the raster import options tab The raster image should replace the DEM. Check to see feature points are where you expect them to be, e.g., mountain tops are located on the DEM high ground

6 Problems with display Band combinations If the colours of the overlay image do not look correct, the R, B & G layers may be displayed in the wrong order Correct using Raster Band Combinations to switch the order of the Red and Blue layers Projection schemes Where the DEM and image overlay have different (or no) associated projection schemes Virtual GIS will struggle, or be unable to display the two in conjunction To solve make sure the overlay image is Geo Corrected under the same map projection scheme as the DEM (see Georeferencing Digital Images)

7 Navigating Virtual GIS Display options View Level of Detail Control Use to define the display resolution of the DEM and the overlying raster image. Consider that high resolutions will effect the speed of navigation Vertical exaggeration View scene properties Lighting and shadow direction View sun positioning Use to produce effect the lighting/ shading of areas of high relief

8 Navigating Virtual GIS 6 options for navigating around a Virtual GIS project are available in a drop down menu within the main toolbar: Dashboard Aerial flight deck. Tricky to use, and not overly useful, but can be used to record flights around a DEM as video files Joystick Only useful if the PC has a joystick attached which it hasn t Position Allows the movement of the whole DEM. Grab the DEM by left clicking and drag it around Selection Used in conjunction with the inquire cursor to read spatial data from the DEM. The other navigation functions are used to position the DEM in the desired location Target Places a red cross in the centre of the image about which the DEM can be rotated in three dimensions Terrain Places the viewer at surface level and allows navigation as if walking over the surface If you loose the image, use View fit scene to window to find it again Of the 6 options Position and Target are the most useful tools for viewing a DEM

9 Saving views Renderings of VGIS scenes may be committed to file using File View to image file

10 Recording a simple DEM fly through Open the DEM scene through which you wish to record a fly through and choose the dash board method of navigation Reduce the window size to reduce the size of the fly through movie Click on the camera button and define a location to save the movie file Choose.avi or.mov,.mpeg is not supported by this version of ERDAS. Click OK Fly through the DEM using the dashboard. The path will be recorded. At the end click on the camera button again to save the results to disk Note. The file size will be enormous for all but very short flights. It is advisable use another utility to convert the movie file to an.mpeg

11 Extracting spatial Data The xyz position of features that can be identified within the DEM overlay may be measured directly using the inquire cursor Open the enquire cursor Utility enquire cursor Position the cross over the feature to be measured Read off the xyz values, or image position values displayed. Unfortunately there is no automated method of recording sets of collected xyz data