The data for Practical 2 is available for download at the dropbox link embedded in the I sent you.

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1 EGM310 UNDERWATER REMOTE SENSING PRACTICALS RORY QUINN PRACTICAL 2: MBES BACKSCATTER DATA Aim of practical To gain confidence in interpreting backscatter data derived from multi-beam echo-sounder surveys, to continue developing your ArcGIS project for the URS assignment and to export maps from ArcMap. Learning outcomes By the end of this practical you should: have developed the basic skills to interpret backscatter data; know how to generate backscatter mosaics in ArcMap; subjectively and objectively segment backscatter mosaics; know how to export maps from ArcMap for inclusion in your assignment. EXERCISE 1: DOWNLOAD AND EXTRACT THE DATA The data for Practical 2 is available for download at the dropbox link embedded in the I sent you. This ZIP file contains numerous files, of which the important data file is: mbes_2m_bs.img: 2m resolution multi-beam backscatter (raster data) 1. Download the file zip to your working folder (the one you created last week) on your external hard drive (HD). 2. Extract the contents of the zip file to the folder. You now have all of the data required to complete the URS. EXERCISE 2: SEGMENTING BACKSCATTER DATA A SUBJECTIVE APPROACH 1. Load the backscatter data (mbes_2m_bs.img) if not already loaded. It automatically loads as a greyscale raster. 2. Change the colour palette so that high backscatter values are black and low backscatter values are displayed as white. It should look like this: [1]

2 3. Note that the areas with data holes (or data gaps) are where the data is of poor quality and so was not used to generate this backscatter mosaic. 4. By selecting the Go to XY icon and changing the map scale: Go to: at a scale of 1:14,000 This brings you to a portion of the seafloor in Church Bay which should look like this: (a) How many distinct acoustic units can you recognise here (e.g. 2, 3 or 4)? (b) How would you describe each of these units in terms of their backscatter signature (high backscatter, medium backscatter, low backscatter, lowmedium backscatter etc.)? (c) Can you spot the wreck(s) in the data? (d) Now toggle on and off the bathymetric data can you see the advantage of having the backscatter data in addition to bathymetric data to aid seafloor mapping? 5. The results of a grab sample exercise in the area are listed in Table 1, can you see think of how you could use the combined backscatter and grab sample data to derive a geological map of the seafloor for this site? How dot he substrate types relate to the backscatter signatures recorded above? Think about grain size and roughness. Easting Northing Sediment type Fine sand Fine sand Sand Sandy gravel Gravelly sand Table 1: Results of grab sample exercise conducted in Churh Bay, Rathlin Island. [2]

3 EXERCISE 3: SEGMENTING BACKSCATTER DATA AN OBJECTIVE APPROACH One of the big issues with segmenting/mapping the backscatter data using the approach in exercise 1 is that that it is based very much on subjective interpretation of the backscatter data. As scientist, we should always aim for an objective approach to mapping. One way to achieve objective segmentation of backscatter data is to use multivariate analysis to categorize the backscatter data. The goal of such classification is to assign each cell in a study area to a defined class or category. Two types of classification are possible in ArcGIS: supervised and unsupervised. In an unsupervised classification, you do not know what features are actually at any specified location (you can establish this at a later stage when groundtruthing the data to generate data such as that presented in Table 1), but you want to aggregate each of the locations into one of a specified number of groups or clusters. What determines to which class or cluster each location will be assigned is dependent on the multivariate statistics that are calculated on the input backscatter raster. Each cluster is statistically separate from the other clusters based on the values for each layer of each cell within the clusters. 1. In the ArcToolbox, go to Spatial Analyst Tools > Multivariate > Iso Cluster Unsupervised Classification. 2. Select the backscatter layer as the input raster layer and change the number of classes to 3. Select OK. I recommend 3 classes as we have previously collected data in the area to supervise the classification and 3 is a representative number of classes. 3. Wait until the new raster layer is rendered (it will take some time) this will be the results of the unsupervised (objective) segmentation of the backscatter data. Change the colours of each class (class 1 to yellow, class 2 to blue and class 3 to green). Now the area around Church Bay should look like this classified map: 4. Zoom to the hull extent of this layer and you will see that the entire backscatter mosaic has been objectively classified using this scheme, and should look like this: [3]

4 The yellows correspond to fine sand, and the blues and greens correspond to mixes of gravel and coarse sand. EXERCISE 4: Exploring common signatures in the backscatter data You will now re-visit each of the sites you looked at in practical 2 this time examining the backscatter signature of the sites you looked at in the bathymetric data previously. 1. By selecting the Go to XY icon in the tool bar, examine and interpret the features at the following co-ordinates. It may help to switch on and off the illuminated bathymetric layer to aid your interpretation and compare it to the signatures you recorded in the previous practical: Go to: Go to: Go to: EXERCISE 5: CREATING AND EXPORTING MAPS FOR THE ASSIGNMENT Once you have created your map, you have a number of choices for exporting it. I find the most effective format is to export as a PNG file. PNG is a versatile raster format that can be displayed in web browsers and inserted into other documents. It supports 24-bit color and uses a lossless compression. For maps, PNG is often the best raster format, since the lossless compression keeps text and line work legible by preventing the compression artifacts that can occur in JPEG format. PNG files also have the ability to define a transparent color; part of the image can be displayed as transparent in a web browser, allowing backgrounds, images, or colors to show through. Additionally, PNGs exported from the data view in ArcMap can be generated with an accompanying world file for use as georeferenced raster data. In this exercise, you will use the map export option to create a series of practice maps which will prepare you for exporting maps for you assignment. Map 1: Exporting a single layer The first map you will produce is a bathymetric map with all of the necessary map elements. 1. In the Data View, switch on your bathymetry and hillshade layers. 2. Switch to Layout View and you should have a screen that looks like this: [4]

5 3. Now add the additional map elements: north arrow, scale bar, and legend using the Insert option from the toolbar. 4. Now add a grid/graticule to the map (right click the map and select Properties) and edit your map until it looks like the one below. Note when you create a legend and want to edit the legend, the easiest way to do this is to convert the legend to a graphic and ungroup it to edit specific elements. This is done by right-clicking the legend and ungrouping. 5. Export the map as a 300dpi PNG file. It should look like this: Map 2: Exporting multiple layers 1. Now add context to your map by adding satellite imagery of the terrestrial component of the study area. To do this, switch back to Data View and go to File > Add Data > Add Basemap. Select Bing Maps Aerial and choose Add. Wait until image renders. Now play about with the map (zoom in and out) to see examine the terrestrial and coastal environments. Of particular interest will be Church Bay on Rathlin Island and Ballycastle where the underwater cable is to make landfall: [5]

6 6. Now, export a new map containing the new satellite imagery layer it should look something like this: Including maps and figures in your report [6]

7 All maps and figures in your report should be numbered in order of appearance and should include detailed figure captions of sufficient detail to stand alone from the main text. An example is provided below. Figure 1 Bathymetric map of the study area off the north coast of Ireland. Bathymetric data, at 1 m resolution, is derived from multi-beam echo-sounder surveys as part of the JIBS project. FINAL REMARKS You should now have developed all of the skills necessary to complete the URS assignment for EGM310. THE REPORT See the module support site for guidance. [7]