Image interpretation in ArcGIS Image Analysis

Transcription

1 Image interpretation in ArcGIS Image Analysis Aim: To explore the image analysis tools in ArcGIS Imagery: Landsat ETM+ multispectral and panchromatic images of southern Tenerife, Spain, acquired on 16 August The multispectral image has a spatial resolution of 28.5m and the panchromatic image has a resolution of 14.25m. The images have been projected to UTM Zone +28. Files: tenerife_south_etm+_ms.img, tenerife_south_etm+_pan.img Data path: W:\bes\student\geoimaging and geoinformatics\data\tenerife\tenerife_south Time: 60 minutes Preparation Copy the data to a folder on your H: or usb device. Launch ArcMap and connect to that folder using Arc Catalog. The Image Analysis Window in ArcGIS Launch ArcMap and open Image Analysis from within Window on the toolbar. Once the window has opened it can be pinned to the sidebar alongside other windows. Notice that the window has three sections: At the top, we have the list of image files that have been opened. In the middle we find tools to control the display. Below this are tools to process imagery. Just beneath the window title there is a button to open Image Analysis Options. Open this window and write down the names of the options. Which option allows you to specify a Number of standard deviations? What does this mean? Display a multispectral image Add (drag and drop) the multispectral image file (tenerife_south_etm+_ms.img) to ArcMap. Examine the Properties of the multispectral image in the Table of Contents. What are the 3 ways in which an image can be displayed?

2 Which technique enables you to display multiple bands of image data in one layer? Contrast enhancement of a single band Use Stretched to display the very near-infrared band (ETM+ band 4 tenerife_south_etm+_band_4). Use the Standard Deviation algorithm and control the contrast enhancement by varying the number of standard deviations. Set the number to be 10 and then change it to 1 standard deviation. What is the effect? Finally, choose a number that allows you to view variations in land cover. What number of standard deviations did you apply to the contrast enhancement? Note: there is no correct answer to this question. It is important that you learn to control the harshness of the contrast enhancement in order to enhance particular phenomena in the image. Contrast enhancement of multiple bands Select the RGB Composite option from the Symbology (in Properties) of the multispectral image. Create a True Colour Composite by displaying 3 images acquired at visible wavelengths: display the blue spectral reflectance (tenerife_south_etm+_band_1) through the blue channel on the monitor, display the green reflectance (tenerife_south_etm+_band_2) through the Green channel and display the red reflectance (tenerife_south_etm+_band_3) through the Red channel. Display Channel blue colour gun ETM+ band 1 green colour gun ETM+ band 2 red colour gun ETM+ band 3 Image Band This is special case of a Colour Composite and it mimics the way an astronaut views the world. Describe the main geomorphological units of the island. Can you identify the flanks of an ancient volcano and the rim of the caldera, ravines that have eroded into those flanks and a relatively narrow coastal plain created from coastal and pyroclastic deposits?

3 Let us now explore what else we might see in images acquired at non-visible wavelengths. Create a False Colour Composite from the very near-infrared reflectance (ETM+ band 4), the near-infrared reflectance (ETM+ band 5) and the red reflectance (ETM+ band 3) as an RGB Composite. Adjust the number of standard deviations to give the optimal enhancement. Display Channel blue colour gun ETM+ band 3 green colour gun ETM+ band 5 red colour gun ETM+ band 4 Image Band Identify half a dozen colours that dominate the composite display? Each colour is related to combination of topography and land cover. Which types of land cover do these main colours relate to? Edge enhancement by spatial filters As well as enhancing the spectral appearance of an image there are image processing tools to 'sharpen' the image. These are only cosmetic changes they do not add information but when used wisely they can assist in visual image interpretation. Add the panchromatic image as a layer (tenerife_south_etm+_pan.img) and change the palette to Stretched, applying the same contrast as used previously in terms of the number of standard deviations. Zoom in on the Tenerife Sud Airport. You should be able to identify the airport in the southeast corner of the island from its' distinctive runways. Open the Image Analysis window and select Sharpen from the pull-down menu. Click the icon to run the process and a new 'sharpened' image will be created that has enhanced the edges of the runway and the motorway connecting the airport to the major tourist centres. Can you interpret the type of leisure facility about 2.5 km to the west of the end of the runway? If in any doubt then switch the display to the false colour composite and examine the phenomena showing up in an orange appearance.

4 Multisource image interpretation You have created colour composites and enhanced the sharpness of Landsat ETM+ imagery. Your final task is to combine these techniques to analyse some features that we have not indentified previously. Quickly refer back to your notes on image enhancement and geomophological interpretation. Create a false colour composite using Landsat ETM+ bands 7, 5 and 1. Zoom to Tenerife Sud Airport. Gradually zoom out in steps until you can identify a number of small 'yellow' patches. What do you think these topographic features might be? Continue to zoom out. Describe the distribution of these features. Are they randomly distributed? Are they evenly spread across the region? Is there any relationship to the geomorphology and/or vegetation cover? An interpretation of these features can be found at the end of the exercise.

5 Answers to Questions in Text 1. Image Analysis Options NDVI Pan-sharpening Orthorectify Hillshade Stretch this includes a setting for the number of standard deviations. This value controls the level of constrast in the displayed image. The default number is 2.0 as this approximates a 95% confidence interval for a normally distributed data set. Increasing the number of standard deviations reduces the contrast in the image and decreasing the number produces a harsher appearance. 2. Properties Stretched - ArcGIS displays a single band that has been contrast enhanced RGB Compsite ArcGIS displays data from up to 3 bands through the red, green and blue colour guns of the monitor to create a colour composite. As such you can display data from multiple image bands in the same layer. Colormap ArcGIS displays a single raster layer and colour codes each grid cell (pixel) according to its' attribute (DN) value. 3. Land cover types The main land cover types in the study area appear in different colours in a 453 RGB Composite: built (urban areas, roads, airport) light grey (sometimes with a pink tinge) vegetated fields (irrigated) - red bare fields or fields under massive polytunnels (greenhouses) grey and white pine forest - brown shrub and grass - green 4. Edge enhancement There are at least 2 large golf courses west of Tenerife Sud Airport. Amarilla Golf and Gold del Sur. 5. Multisource image interpretation The small hills are cinder cones. They are concentrated in an area known as the Bandas Del Sur that forms the south-east flank of the ancient volcano. They reflect strongly at shortwave and near-infrared wavelengths and show up as 'yellow' if ETM+ bands 5 and 7 are displayed through the red and green colour guns respectively. Areas of similar colour can be found close to the caldera rim which might suggest that they consist of similar materials ejected by volcanic activity.