USING CCCR S AERIAL PHOTOGRAPHY IN YOUR OWN GIS

Transcription

1 USING CCCR S AERIAL PHOTOGRAPHY IN YOUR OWN GIS Background: In 2006, the Centre for Catchment and Coastal Research purchased 40 cm resolution aerial photography for the whole of Wales. This product was flown in While you are a student at AU, you are welcome to use this data for your own coursework or research. Otherwise, this and similar data are commercially available from GetMapping (among other vendors). Purpose: To load georeferenced aerial photography from within Wales at your study site into your own GIS map using ArcGIS. This tutorial assumes you have a rough understanding of what GIS is but no experience using ArcGIS. STEP 1: Finding what data you need The aerial photograph tiles are already georeferenced onto the British National Grid coordinate system and are available in 1km 2 grid tiles. The naming convention for the photographs is based on the Ordinance Survey 1 km National Grid, which can be read off any OS map. If you know the tile names (e.g. SM6422), you can skip ahead to Step 2. Otherwise, you will need to identify which tiles you need. There are various ways to do this. Here, one workflow is shown that relies on loading the 1km national grid and a OS map into ArcGIS to provide some spatial context to identify your sites. Getting OS National Grid Tile Names To acquire the 1 km OS National Grid, you can use Digimap ( 1. Go to Edina ( Click on Login. You will be routed through a UWA log in. Just use your UWA username and password. 2. Click on Ordnance Survey Maps. Click on Data Download Services. Click on Data Download. 3. Click on Boundary Download. 4. You will see a variety of UK boundaries that you can download in various formats. You can download any of these for context (e.g. national boundaries), but if you are using the data in ArcGIS, make sure you download it in a *.SHP shapefile format. For this exercise, select *.SHP under GB National Grids and then hit Continue. 1 of 13

2 5. You will be given four area choices for the grid scale you wish to download. For this exercise, download the 1KM National Grid Squares. Click Continue. 6. On the next screen you will be asked what format you wish to download the data as. So long as you have a way to unzip files, I suggest using the *.zip format default. Click on Extract Data 7. It may take a couple minutes or so to process your request. Just follow the instructions, then download your data.zip file. 8. Extract the data from the zip file to a known location (preferably to your own filespace, computer or external hard drive). 9. You now have a vector (polygons) shapefile for the 1KM grid for the whole of the UK you can view in ArcGIS! Getting an OS Base Map for context We now need to get a map so we can find our site. We will download the data in a raster (grid) image format (*.tiff) again from Digimap. 1. Go back to the Digmap homepage. Navigate to Data Download Services and Data Download. 2. In the step 1 screen, you can choose any type of base map you wish. I would suggest using the OS 1:50,000 Scale Colour Raster in an uncompressed TIFF format (this is the Pink Land Ranger OS map series). Click continue. 3. In the step 2 screen, you are given a variety of means to find the location for which you want to download your map. In this example, lets focus on the confluence of the Rheidol and Ystwyth Rivers with Cardigan Bay here in Aberystwyth. Thus, you could just do a Placename search for Aberystwyth. 4. In the step 3 screen, your search results are presented. Select the desired result and continue. 5. As is very often the case, you re site of interest happens to fall on the boundary of four different map tiles! You can either select the tile(s) you want from the list or select them on the map. For this example, select SN48, SN68, SN46, SN66, and SN68. Click Continue. 6. Again, download the data as a zip archive. This can take some time if you ve selected multiple tiles. 7. Extract the data from the zip file to a known location (preferably to your own filespace, computer or external hard drive). A Note about your File Management With GIS analyses, you can quickly produce or acquire tons of files and it is very easy to loose track of what information is what. ArcCatalog (part of ArcGIS) is a file manager (similar to Windows Explorer for My Computer) specifically for GIS data. You can organise your data in anyway that is logical to you, but I would strongly urge you to use the metadata features in ArcCatalog to record what files or directories contain what. I would also suggest placing your data in a project folder (that can be referenced from multiple maps) and organised into subfolders for 2 of 13

3 different data types. For example, perhaps you might have an Aberystwyth Folder, and in it create three folders for now, one called OSBaseMaps, one called MyMaps and one called APs (for aerial photos). Most of the data you download will already have some logical filenames, and it probably makes sense to keep these. You will have to develop your own naming convention for the files you create. Just be consistent and stay organised. Also, don t move or rename files around if they are referenced from a map and you want the reference to be resolved (you can always repair data sources to fix this, but easiest just to be organised ahead of time). Assigning Coordinate Systems for your Data ArcGIS does not know what coordinate system your data is projected in unless you tell it. For both data you download and data you create, you need to define the coordinate system. This can be done within ArcMap by right clicking on the loaded data and adjusting the properties, or within ArcCatalog. It is good practise to define the coordinate system you are working in for your datasets in ArcCatalog before you load it into a map. 1. Once you have the data where you want it open ArcCatalog and navigate to the data. 2. You can set the coordinate system for rasters or shapefiles individually by right clicking on the data set and clicking on properties. For rasters: You go to spatial reference, select edit and then load the desired coordinate system. For shape files, do the same, but then click on the XY coordinate System Tab. 1 1 Note this tutorial assumes you are using ArcGIS 9.2. The procedure is slightly different in older versions. 3 of 13

4 3. If you have a bunch of data sets, you can turn on the arc toolbox window, navigate to Samples -> Data Management -> Projections -> and double click on Batch Define Coordinate Systems. This will bring up a dialog in which you load all the data sets to define the coordinate system for, select the coordinate system and do them all in one go. 4 of 13

5 Loading the Base Map and Grid in ArcGIS When you start Arc Map, you will create a new empty map. Remember that data is not stored in your maps, simply referenced in maps from where ever you stored it (see previous subsection). If you will be viewing your data and maps on different machines, there may be different paths. As such, it is wise to change your Map Document Properties to store relative path names (so that as long as path relative to the Map is the same, it will work on any machine). To do this, go to the File Menu, click on Map Properties, then click on Data Source Options and change the appropriate setting. Save your Map in your project directory or some other logical place. 5 of 13

6 When you open ArcMap, make sure that the 3D Analyst, Spatial Analyst, Georeferencing and Editor toolbars are loaded, by right clicking on menu bar area and selecting the appropriate toolbars. 6 of 13

7 Now we re finally ready to load the data you downloaded. 1. First, manually define the coordinate system we wish to use for the data frame. Right click on the Layers data frame in the layer dialog on the left hand side. Click on the coordinate system tab and select the British National Grid, under Projected Coordinate Systems, National Grids. 7 of 13

8 This will mean that all data you load into this data frame is assumed to be georeferenced within this coordinate system (OS National Grid). The default otherwise is to just use the coordinate system of whatever the first data set you add to the map is. 2. Click on the add data button from the main toolbar and add your Base Map raster images first (SN68.tiff, SN46.tiff, SN66.tiff, and SN68.tiff). 3. The map window should automatically resize to fit these on the screen. If it does not you can use the zoom tools or highlight all the layers, right click and select zoom to layer. 4. It is helpful to organise your layers so you can keep track of them by creating layer groups within the data frame. For example, you can select all four of the layers you ve loaded, right click and then select Group. This creates a new group with all these layers inside of it. You can rename this group something you recognise, for example OS Base Maps. Then when you want to turn on off these layers or change their properties, you can apply this to the whole group. 5. Next add the 1 KM grid layer using again the add data button. This will probably now obscure the base map data. You can drag this new layer underneath the OS Base Map group, but then you can t see it. If you put the 1 KM Grid back on top of the layer list, right click and select properties for this layer. Go to the Symbology tab and under the symbol shown, click on the symbol to change its view. This will bring up a symbol selector dialog: 8 of 13

9 If you select hollow, this will change the fill colour to transparent so that you can see the grid outline, but also see the map underneath it. 6. If you navigate to Aberystwyth (using the pan and zoom features), you will notice that the 1 km square grid was actually already on the OS 1:50000 basemap you downloaded (turn on and off the grid layer to convince yourself). 7. However, we still don t know what the name of the individual grids are (this is the whole point of this step to get these tiles so we can acquire the APs for our site of interest at the confluence of the Ystwyth and Rheidol). Right click on the 1 km grid layer and Open the Attribute Table. You will see a table with three columns, the third column contains the name of the tile. 8. If you click on the Identify button, this will bring up an identify dialog. You can then select an area of the map you are interested in and it will tell you the properties for the selected layer: 9 of 13

10 In this example, the grid tile name is indicated by the PLAN_NO SN5881 (where SN is the grid region, 58 is the easting and 81 is the northing, corresponding to a coordinate of , ). You can manually go through and select all the tiles you are interested in and right down the numbers. 9. Alternatively, if you again click on the properties of the 1 km Grid layer, this time navigate to the labels tab: If you click on the label features in this layer, and make sure the label field is changed to PLAN_NO, this will place a label on every grid cell. You many need to adjust the colour and/or font so you can see this. 10. All that you need to do now is make a note of the tiles you need by writing down the corresponding PLAN_NO. STEP 2: Locating the Data on the IES Server: If you are on an IES domain machine 2 (e.g. F-Floor Lab), you can navigate through My Network Neighbourhood 3 to find the IES domain (under Microsoft Windows Network), and then the IES file server, called Iessvfile01. 2 If you are not a member of the IES domain, you will not have access to an IES domain machine. You can submit a request for the data by completely following step 1, contacting Joe.Wheaton@aber.ac.uk, and sending a list of the requested tiles. If appropriate, you may request that you are added to the domain to access the data directly. 10 of 13

11 Click the + next to Iessvfile01 to see the contents: 3 Note, when you are navigating in My Network Neighbourhood, use the + symbols to expand the tree as opposed to clicking first on the computer or folder itself (it loads quicker). 11 of 13

12 If you navigate down to \\Iessvfile01\RBDH_NO_BACKUP\Research\Data\Wales Data\Air Photos\Wales 40cm you will find a series of subdirectories with two letter names that correspond to the OS national grid reference. 1. To get your photos of interest, simply navigate to the appropriate directory and scroll down the list to find the photos. 2. Make sure you copy both the *.jpg and *.jgw file to your local working directory! Without the *.jgw file, ArcGIS will not have the necessary information to georeference this file correctly! 3. Before you load the files in ArcGIS, you should repeat the steps from above of assigning a coordinate system to the images in ArcCatalog. 12 of 13

13 Step 3: Loading the Aerial Photos The final step is simply to load your aerial photographs into ArcGIS and verify that they are in the correct locations. You use the same Add Data feature as before. Additional References or Further Reading: 13 of 13