Downloading and formatting remote sensing imagery using GLOVIS

Transcription

1 Downloading and formatting remote sensing imagery using GLOVIS Students will become familiarized with the characteristics of LandSat, Aerial Photos, and ASTER medium resolution imagery through the USGS Global Visualization Viewer (GLOVIS) at This utility can also be used for acquiring imagery (the imagery used in this exercise has already been downloaded to reduce processing time). Introduction to the GloVis interface Landsat 1. In order to become familiarized with procedures for accessing imagery from USGS s GloVis website, follow the steps and answer the questions related to LandSat imagery located. a. Open GloVis ( in a web browser b. Locate southeastern Virginia by clicking on the interactive map. Next, enter the coordinates of the Fort Story Officer s Club (36.926, ) to more precisely locate the study area. c. From the GloVis viewer, set Collection to the imagery of interest (LandSat Archive - L4-7 Combined); Note: Data descriptions provide valuable information for each platform, including available bands. d. Click on the images shown until you have selected the most recent cloudfree image that covers Fort Story. The box labeled CC indicates the percent cloud cover for each scene. Next and Previous buttons allow you to access successive scenes. e. Right click over the image and View metadata; click FGDC metadata for added details. Landsat file naming convention follows. Using GLOVIS Page 1

2 What does the metadata reveal for each of the parameters below? Complete table with your responses. File Name Path and Row Acquisition date (m/d/y) Spacecraft identifier Bands listed LandSat You can download the selected image, by clicking on Add and then Download at the bottom left of the viewer. Do this now to see the following graphic. For the sake of time, please select No. 2. This file has already been downloaded for you and can be found in compressed format in your data directory with a the following filename: LE EDC00.TAR.GZ 3. In order to be used, the files in this compressed directory must be extracted. To extract the files, you can right click on the zipped file and select Extract, choosing the option to extract the files to your working directory. To save time, I have already extracted these files for you. Let s take a look closer Using GLOVIS Page 2

3 look at these files to understand their download format. 4. Open ArcCatalog. 5. Navigate to the Glovis_Landsat directory and explore the contents. Note that when the downloaded file is unzipped, it produces 11 different files, 9 raster images and 2 text files. The raster images correspond to the different spectral bands provided by the Landsat ETM+ sensor. 6. In the directory view, highlight one of these images and then click on the Preview tab in the display view. When asked if you would like to build pyramids, select No. Note: Pyramids are copies of the original bands of data where the spatial resolution has been consecutively reduced by a multiple of 4. They improve draw speed for raster datasets displayed at less than their full resolution. To be useful for multispectral analysis, these images must be stacked into a single composite image comprised of multiple bands. More on this in a later exercise. National Aerial Photography Program (NAPP) and High Altitude Photography (NHAP) 1. In the GloVis viewer, change the Collection to Aerial NAPP. 2. Re-center your viewer on Cape Henry by entering the coordinates, Lat , Lon Using GLOVIS Page 3

4 You may Add and Download this file using the tools at the bottom left of the viewer. This step has been done for you. 3. Open ArcMap and add the date element continent.sdc for background purposes. 4. Two air photos have been added to your course directory in the Glovis_Photo folder. Review this folder in Windows Explorer and you will see two files with the.tif extension. 5. Add the file entitled NAPP_CHenry to your ArcMap display. When prompted to Build Pyramids, select YES. ArcMap will generate the pyramid files necessary for rapid display at varying resolutions. 6. Before viewing this file in ArcMap, let s take another look at the Glovis_Photo folder. We now see two additional files with the same name, but with.rrd and.aux extensions. These are the additional files created by the pyramid building process. **These files will NOT appear in the add data browser in ArcMap because ArcMap understands that they are components of a single data element 7. In ArcMap, review the display. The NAPP_CHenry image will have been placed in a strange location on the world map. Why is this? Examine the Properties of the NAPP_CHenry layer. Using GLOVIS Page 4

5 What is the Spatial Reference for this layer (Source tab)? 8. Right click on NAPP_CHenry and click Zoom to layer. What are the obvious problems with the placement of this image? This misplacement is due to the fact that these photos are image files without any spatial reference (much like family photos in your photo album). To be used for geospatial analysis, they first must be georeferenced (something that we ll do in a later exercise). 9. Click the Full Extent icon on the ArcMap menu to zoom back to the world map. 10. Add the NHAP_CHenry image to the display, building pyramids as you do so. 11. Zoom to this image. Where is it located in relation to the NAPP Cape Henry Image? When any data element is loaded into ArcMap, the software tries to make sense of that element in relation to the first element loaded into the viewer (continents.sdc, in this case). Using GLOVIS Page 5

6 Although these two images are not georeferenced, ArcMap places them in the best location based on the available information encoded in the files. **As you can see, data placement may be totally incorrect. It is critical that the user examine every data element for logical consistency and proper placement. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) ASTER is an imaging instrument flying on Terra, a satellite launched in December 1999 as part of NASA's Earth Observing System (EOS). ASTER is being used to obtain detailed maps of land surface temperature, reflectance and elevation. 1. In the GloVis viewer, change the Collection to ASTER ASTER L1B VNIR US Rolling Archive. Access to the L1A (Level 1 A) data is currently restricted. 2. Click on the tile covering Cape Henry. Once this tile is highlighted, you may Add and Order. To save time, this download has already been performed. 3. ASTER imagery is not downloaded directly from GloVis as is the case with Landsat and aerial imagery. When clicking the order button, you are directed to a website to complete the process and are notified via link when your data is available. Below is a brief review of screen shots for this process. Using GLOVIS Page 6

7 4. You will receive an that contains instructions for downloading your file from the USGS ftp server. 5. Open a new (blank) ArcMap document and add the date element AST_L1B_ _ _6484.hdf, the only element within the GloVis_Aster folder. ASTER file naming conventions Downloaded file in hdf or heirarchical data format: AST_L1B_ _ _17127.hdf [sensor_level_processing id_date of capture_date of processing_scene id ] Note: Hierarchical Data Format (HDF) is a data file format designed by the National Center for Supercomputing Applications (NCSA) to assist users in the storage and manipulation of scientific data across diverse operating systems and machines. 6. ArcMap will present a message allowing you to select which bands you would like to display. The ASTER instrument consists of three separate instrument subsystems: visible & near infrared (vnir), short wave infrared (swir), and thermal infrared (tir). Bands 1-4 are VNIR and have a 15m spatial resolution. Bands 5-10 are SWIR with a 30m resolution. Bands represent TIR, with a 90m resolution. For this exercise, we ll display only the VNIR bands. 7. Select subdatasets 0-3 in the menu by holding down the shift key while making your selections. Click OK. Although you ve elected to display all three visible bands and the NIR band, ArcMap does not automatically generate a composite image that allows you to alter the color bands in a single image. Instead, ArcMap merely imports the bands as separate panchromatic layers within a group layer. Using GLOVIS Page 7

8 To make this ASTER imagery more useful for spectral analysis, you ll need to generate a composite layer stack (something which we will review in a later exercise.) Using GLOVIS Page 8