Lab exercise 9, due right before
the class on November 2, 2005
EES5053: Remote Sensing. Earth and
Environmental Science, UTSA
http://www.utsa.edu/LRSG/
Student Name: ___________________
DEM and create 3D visualization using ENVI
Purpose
In this lab, you will get to know what the digital elevation model (DEM) image looks like and how to download them from Internet. Two DEMs you will use are Shuttle Radar Topography Mission (SRTM) DEM and National Elevation Dataset (NED) DEM. Then you will know how to draping the Landsat ETM+ image and GIS layer over the DEM to create a 3D perspective view. Below is an example I did and you will do it.
3D perspective view of San Antonio
Step 1. Preparation
Create a directory Lab9 under /EES5053, or simply rename the Lab4 (or Lab5) to Lab9, since you will use the same image and GIS layers you used there. Create Data and MyWork subdirectories under Lab9 if you create a new Lab9 directory.
Step 2. Download the DEMs from Internet
(notice: you may ignore this step and directly go to Step 3, if you do not want to practice how to do this step, since I have already did this for you and you can use the data directly for the step 4. It will take some times for you to get familiar with this downloading system and import to ENVI system. But it is definitely encouraged to do it in your own time if you think you will use it for your class project or later on. If you have any question during the downloading, please ask me any time)
From the lecture, we know InSAR technology has been used to derive the DEM and to study the surface displacement. In this lab, you will download the 30 m SRTM DEM derived from InSAR tech and 30 m NED DEM converted from 1:24,000 national topographic map. Please go to the USGS seamless distribution system at http://seamless.usgs.gov/. Click “View and Order Data Sets - United States Viewer”. From the right hand column, you will see “Download layers”. Under Raster, you will see the NED and NLCD are checked (default). Check the 1/3” NED and SRTM 30m, select the San Antonio area. When a new window pop-up, you need select the Geotiff format to save. So it can be import to ENVI.
Step 3. Import GIS layer to ENVI
If you skipped the Step2, you can download the two DEM images from here to your Image directory.
Launch ENVI and load the ETM+ image with RGB as R-4, G-3, and B-2 compositions or others you like. Then overlaying the GIS layer to your image. If you forgot how to do it, refer back to Lab5. You can change the color of GIS layer, you can edit, query attributes, and you can create new GIS layers by drawing lines, polygons.
Play with the image and check how the roads match the roads from image. You will find they are actually match very well.
Step 4. Load two DEMs and make comparison
Load NED-SA.tif and SRTM30m.img from NED30m and SRTM30m directories, respectively. Before you actually load them into a display view, please check the Map Info of the three of images (Two DEMs and one Landsat ETM+ image) under the Available Bands List window.
Question 1
Please record these map information into your homework. Discuss the difference of them, and transfer the 0.000278 degree to meters. Show your work.
Question 2
Load these two DEMs to two new display windows. Link these two. What do you think you should use to link them: Link Displays or Geographic Links? Which one is right after you tried? Why?
What the value of each pixel represents in those DEM images?
Question 3
Show the statistics of these two DEMs, and explain what you can conclude from the statistics. The unit of the DEMs is meters.
Step 5. 3D perspective view
From the main ENVI menu, click Topographic-> 3D Surface View, select Disply#1 (the ETM+ image), click OK. In the Associated DEM Input File window, select either the NED DEM or SRTM DEM as input, Click OK. A new window Called 3D SurfaceView Input Parameters opened. Check the Full for Image resolution, change the Vertical Exaggeration 15, check Nearest Neighbor for Resampling. Click OK. Now you should see the 3D SurfaceView window. You can enlarge this window to a larger one.
In this window, click Options -> Import Vectors. You will see the roads are overlaying on the top. Hold the left button of the mouse, you can rotate your view, hold the right button to zoom in/out, or hold the middle one (if you do not have a middle one, then the ctrl+left will work the same way as the middle one), you can move the view to any direction. You can save your 3D as an image for presentation or sending to somebody else.
Click Options-> Motion Controls, you will open a new window. This window can let you Add sequence views, then Play Sequence. You can design an Annotation Fly Path, let the 3D view flying through. Then you can save your sequence and fly through, or save as a MPEG (it seems not work now).
Question 4
Paste one of your 3D views to the homework such as the one I did.