Lab 3 -- Mosaic. Introduction

Transcription

1 Lab 3 -- Mosaic Introduction You will be introduced to image mosaic. In the first part, detailed steps are given to mosaic two images (mosaic_1.img and mosaic_2.img). These images are digital elevation data (DEM) of Colorado Springs, CO. The brightness of the image shows the elevation. The bright areas are high in elevation, and dark areas are low in elevation. Then you will mosaic 4 quadrangles DEM data of Greenville obtained from U.S. Geological Survey (USGS). Lastly, you will mosaic TM8 images (990923_gv1_b8.img, _gv2_b8.img, and _gv3_b8.img) covering the floodplain along the Tar River between Greenville and Washington, NC. In mosaicing, images must be geo-referenced, and have the same pixel size (or resolution) or resampling of the data will occur as Imagine does in mosaic. Part 1 Viewing the images Start Imagine and open two viewer windows to display mosaic_1.img and mosaic_2.img. Click on the i icon to open the ImageInfo dialog boxes, respectively. Figure 1 shows the image information of mosaic_1.img. The Upper Left X and Y, and Lower Right X and Y coordinates, pixel size, and projection of the images are the basis for mosaicing the images (in the sections of Map Infor. and Projection Infor, Fig. 1). Answer question 1. After done, close the ImageInfo dialog boxes by clicking on the File/Close or x on the upper right corner (Fig. 1). Fig. 1. 1

2 Fig. 2. Mosaic Images Click on the Raster/Mosaic/MosaicPro to open the Mosaic Tool dialog box (Fig. 2). Click on the Edit/Add Images or the 4 th icon on the 2 nd row to open the Add Images for Mosaic dialog. Navigate and locate folder part1 of this lab. Click on the mosaic_1.img, and then on the Add. Click on the mosaic_2.img and then on the Add for mosaicing. Click on "File" (Fig. 2) to save the mosaicing model file as mosaic_part1 or any name you would like to give, and make sure that the file saved is under your current working folder (part1). (The open file icon will allow you to navigate to where you want to go to.) Finally, click OK to finish. 2

3 Now, click on the Process/Run Mosaic (Fig. 2) to open the "Run Mosaic" dialog box (Fig. 3). Give a name for the mosaic, and make sure the mosaicked image will be in folder part1 (Fig. 3). Then, click on OK to mosaic. Open a viewer widow to view the mosaiced image (Fig. 4). Answer question 2. Fig. 3. Fig. 4. Part 2 Mosaic of 4 quadrangles of USGS DEM data for Greenville There are four DEM files under folder part2. Use what you have learned to mosaic them, and save the mosaiced file under part2. Answer question 2. However, the result may not be satisfactory; there are gaps in the mosaiced image. The gaps occur where two quadrangles are overlaid. (Welcome to the real world!) The explanation from USGS documents is that each DEM quadrangle was created at a given projection, and then re-projected to the current projection (UTM/Clarke 1866). There were almost no overlaps among original DEM quadrangles. They (USGS) just cut them too tight. Let's fix the gaps next. By checking the pixel values within the gaps, one noticed that if there are no data on the edge of a given DEM quadrangle, the values are set to the maximum value available in the data set. In our case, the value is 255. However, on the adjacent DEM quadrangle, there are data values available. Typically, these real values are less than the maximum value set for the no data area. Given that, we i) click on the Edit/Set Overlap Function (Fig. 2) to open the Set Overlap Function dialog box (Fig. 5). ii) iii) select the "Minimum", and then click on the "Apply". click on the "Process/Run" to finish the mosaic. Again give another name to the output file and save it into folder part2. iv) open a view window to view the revised mosaic image. The gaps will be fixed (Fig. 6). Answer question 2. Part 3 You need to mosaic three TM 8 images. They are stored under folder part3. Copy and paste the three into your working folder before you proceed. Because these images are not geo-referenced yet, you need to reference 3

4 them together. For each image, view it. Open the ImageInfo dialog box, edit following information into the ImageInfo so that all the images will be geo-referenced. (You need to edit the ImageInfo one image at a time with a total of 3 times.) Fig. 5. Fig. 6. Images Upper Left X Upper Left Y Pixel Size X Pixel Size Y _gv1_b8.img _gv2_b8.img _gv3_b8.img i) In the Change Map Model dialog box; type in the Upper Left X and Upper Left Y, set the unit in meters and projection as UTM (from the drop-down option), and click on the Ok to finish, and ii) iii) iv) In the Add/Change Projection dialog box; set the Spheroid Name as WGS 84, Datum Name as WGS 84, UTM Zone as 18, and NORTH or SOUTH as North from the drop-down options. Click on the Ok to finish. Figure 9 shows the information has been changed for _gv1_b8.img. Once you have geo-referenced all three images. Then mosaic them. Name the mosaiced image as tarriver_gv_washington.img. Answer question 2. Questions 1. (1 point) What are the pixel sizes (X and Y) of both images? Based on the X and Y coordinates of both images, can you tell the relative position (up, down, left, right, etc.) of mosaic_2.img to mosaic_1.img? 2. (4 points) Each correctly mosaicked image will be credited for 1 point. 4

5 Acknowledgment This lab handout was supported by an AmericaView grant to the East Carolina University at Greenville, North Carolina, 27858, USA. 5