Georeferencing, Editing and ArcScene

Intro to GIS - Week 13

This lab started with georeferencing two raster images of the UWF campus.  Georeferencing is the method used to tell a raster dataset with no geographically referenced coordinate data built into it where it belongs geographically.  This is done by linking the target layer, the unreferenced layer such as the raster image, to a referenced layer using control points to match up common points between the two.  In this exercise we matched buildings in the raster to buildings in a referenced building layer.  Several points are usually required to create an accurate reference and they should be evenly spread throughout the image.  I found the Image Viewer Tool to be very helpful in placing the control points.

Once the links are places with the control points they can be viewed in the links table.  This table shows the residual value which tells you how much each link agrees with how the layer is currently displayed.  The lower the value, the more accurately the control point is georeferenced. I had two control points that were considerably higher than the rest and when I zoomed in to look at them more closely I saw they weren't that well aligned and there were a couple other buildings around them that were not either.  I deleted those control points and set a couple others.  This made for a much better visual alignment and when I checked the table these residual values were much more in line with the rest.  Once my residuals were more in line and a closer examination of my image was made I felt I had a good match.  I went back to the table to check the RMS (Root Mean Square) Error and saw it was well below 15.  RMSE is frequently used in GIS as an indicator of the accuracy of the spatial analysis and/or remote sensing.  It is a measure of the differences between the calculated values and the actual, or observed/measured, value.  The difference between the calculated and actual value is called the residual.  The RMSE, derived from squaring the difference between the actual and calculated values adding these residuals together, dividing that by the total number of values and taking the square root of the result, aggregates the residuals into a single value.  My RMSE for the north raster of UWF was 4.743 with a 1st Order Polynomial Transformation.  My second raster for the south portion of UWF was much better at 1.66888 with a 2nd Order Polynomial.

The next section of our lab was editing.  In this section we used editing tools to digitize two new features; a building and a road.  To digitize a new feature you must first start an editing session.  This can be done either by clicking on the editor menu of the editor toolbar and selecting start editing or right-click on any layer in the table of contents and select Edit Features then Start Editing.  Selecting Create Features opens a window with the templates in the map in the top panel and tools available to create that type of feature in the bottom panel.  We used the straight segment and endpoint arc tools for both the building and the road, utilizing the snap options on the road to make sure the new road lines met existing road lines and endpoints were placed properly.  Edits are not automatically saved, not even by saving the map.   Before ending the editing sessions with Stop Editing you must first select Save Edits or everything will be lost.

The third section we had to create a multiple ring buffer around an eagle's nest on campus property to show the location of a new conservation easement to protect the nest and the eagle.  First we started with creating a hyperlinked picture of the eagle nest to the attribute data, then through the layer properties tab HTML Popup, choosing to show content as a URL using the picture field.  Next we used Identify to select the point feature in the drawing and clicked on the lightening bolt to verify the link worked.  Next we used the Multiple Ring Buffer tool to create two rings around the protected nest.

Section 4 had us create a 3D view of UWF in ArcScene.  We started with a DEM file of UWF which had elevation data, then draped the other layers over it in the Base Heights tab of each layer's properties and selecting "floating on a custom surface:" and selecting the UWF_DEM to use its elevation data.  Next we used the height field of the buildings layer as our z-value to extend those features above ground by checking "Extrude features in Layer" option in the Extrusion tab of the buildings layer properties. We then set the vertical exaggeration in the scene properties to accentuate the building heights.  With that done we exported the scent to a 2D image to finish off in ArcMap since ArcScene does not allow the addition of map elements.  I had to go back and forth between ArcMap and ArcScene a couple times to adjust my angle so the digitized road would show in the image but there would still be enough relief to show the height of the buildings.  I ended up having to increase the line weight of the road symbol to make sure it showed up.

I really enjoyed the editing portion of this assignment and am looking forward to doing more such work in later courses.