Georeferencing in ArcMap Tutorial

howdy today I will discuss and demonstrate using arcmap to georeference a hardcopy map for use in GIS this is a great way to use information found on older paper maps and integrate that data into your GIS project we will be working with ArcGIS 10.1 for this video there have been some important changes to the geo referencing tool with a 10.1 upgrade but for this work we will do today it will be very similar to version 10 to start you will need to have your maps scanned and saved as a GIS readable format common image formats such as Tiff's JPEGs PNG s and gifts will work I will be using a JPEG for this example if you are do referencing a map you will also want to know the coordinate system and projection information used to create it for many Maps this may not be possible to locate but we advise checking any accompanying information our metadata for projection info for my image I can find the projection information under the title and see that it is using WGS 1984 to set our coordinate system we're going to start by opening a new blank arcmap and we're going to go to the table of contents and right-click on the layers icon and go down to properties and when we click on this it's going to open the properties dialog box we'll go to the coordinate system tab here and if we look in the lower box in this tab we'll see that there is no coordinate system information set for the arcmap document but if we go up to the box above it and expand the geographic coordinate system folder we will be able to set our coordinate system by navigating to the one we want and in this case we're going to use WGS 1984 so we click on that and we'll click OK to set that our next step will be to add a reference map or background you want to choose a reliable reference map that is spatially accurate and displays similar features to the map you are giving a location to for this satellite or ortho imagery can be used streaming base maps or feature classes without lines and streets are also common sources in this example I'm using an outline of the US Coast along the Gulf of Mexico to go with my map I know that this shape file uses the WGS 1984 coordinate system so I know it's going to work well with my scan map because that's the court system that you was used to create the map now we go to add data to load our JPEG image so we'll navigate to where the image is saved and add it to our map document once we do this we will receive a message asking if we would like to build pyramids for the image we can click no and then we're going to receive a warning box telling us there is no coordinate information which we are aware of because we are intending to georeference this image so we can just click OK and by default images that have no coordinate information in arcmap will draw with our top-left corner at point 0 0 so we're not going to see it and so to start we can right click in the gray area at the top and that will bring up our toolbars menu and we'll add the geo referencing toolbar and then I'm going to dock this toolbar up at the top now to get the image where we want it to be we can start by zooming in on our desired area then on the geofencing toolbar will use the grf in scene drop down menu to use the option fit to display and this is going to fill our current view with the entire map and so it's pretty close to where I want it to be now we can start adding ground control points to do this we can use distinct geographic features such as street corners or coastline edges to identify where we would like to place ground control points try to use stable features that will have maintained their location over a long time especially if you're using an older map basically features that are almost permanent if you have a map with coordinate grids on it you can simply click on the location where the coordinate is and add the ground control points by manually typing in the coordinates and I'll go over that later and this is a subjective process it depends highly on the type of map you are using and how much information you have on the map that you want to retrieve from it so let's start adding points to do that we'll need to identify an area where we think would be a good ground control point and so I'll just zoom in on the map here and select a location and then I'll use the ground control point icon click first on the location that's on the scan map and then next to the reference map and then automatically adjust after I add my point now I'm going to continue adding points and it's important to remember that what you're adding points always click on the image first then click on your reference locations now after you have a ground control point you can continue adding them and you'll want to spread out where you add points do not add points in one location it's a good idea to go almost the exact opposite side of the map that way you have a good spread and where the map is geo referencing and this is going to give you a better quality geo referencing to add a point and then enter the specific coordinates when you know the exact latitude and longitude of a point we are simply going to use the ground control point tool and we're going to click once on the known location in this example we have a line scroll on the map and then right click and that's going to give you the option to input x and y and so we know our coordinates and we can just type those into the box here and then click OK after we're done typing and this essentially is telling the point we just added that it's at this exact coordinates rather than clicking back and forth between two points we observe on the map now as we add more more points there's going to be more warping of the image if your original image was not drawn exactly to scale or without any coordinate systems it may be appropriate that there is warping however when you have an image that matches closely to your system they should match relatively well without warping so let's take a look at the view link table on this table you'll see each ground control point you add it appears as a record in the table and at the bottom of this table you'll see a drop-down box that is the transformation drop-down option box which lets you know which transformations are available this table also displays residuals if you look to the right side of the table as you add points these residuals change they are similar to a measure of error for each point you must have at least three ground control points before residuals will be calculated the lower the residual value the better the fit of the ground control point to improve your geo referencing you can delete the points with the highest residuals I'm not going to go much further into detail on residuals and transformations here because it's a lot of information and may not be needed for the simple G referencing that we're doing but it's good to know that these can be manipulated and they do affect the quality of the output of your geo referencing now generally you can add 10 to 20 ground control points when your geo referencing but if your paper map closely matches the projection and coordinate system you picked you may only need three or four ground control points to get the fit that you want be sure to re-examine the fit of your map to the base map or reference map and and then you can start deleting points in your view link table that have high residuals until you get the fit that you want once you're satisfied with the result it's time to start saving so if we go back to the view link table and click on the Save icon we can save our ground control points as a text file this is important if you want to change the geo referencing of your image later you will still have those points that you used available and you can reload them to save your actual raster image file you can go back to the geo referencing toolbar and we have two options update or rectify update creates an additional file called a world file that stays with the image and provides spatial information for it when in the same folder as the image rectify creates a whole new raster image that is worked to fit the geofencing that you've just done it also has companion files that must also be kept with it in the rectified dialog box you will need to give it an output location for the new raster and there are several other options you can choose from as well for the new SAF raster now for update geo referencing we're going to go back to the geo referencing toolbar and simply click on update geo referencing and this is going to create the new world file and do the save automatically and now you are ready to use your image as GIS data be sure to document in your metadata or published work that you use geo referencing for this particular piece of data and all the spatial information on your base map and background image that you used to geo reference it well this concludes our tutorial we thank you for taking interest and for more information or resources from the Texas A&M University Libraries map and jazz library please visit our website at library tomorrow you forward slash maps - GIS

Channel: Map & GIS Library

Views: 422,158

Rating: 4.8870692 out of 5

Keywords: GIS, Georeferencing

Id: PHtxbpboDro

Length: 9min 59sec (599 seconds)

Published: Fri Jan 11 2013