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Lab 4: Advanced Editing and Topology
What Youll Learn: Image interpretation, more editing, and topology. You should read the section on topology in Chapter 2, and Chapter 4 on image interpretation of the GIS Fundamentals textbook before starting this Lab. Data: The several data layers are in the Lab 4 subdirectory, all in the UTM, NAD83(2011) zone 15 coordinate system, for a portion of Big Marine Lake, in Washington County, Minnesota: BigMarSum.img, a summer infrared image, with a 1 meter cell size, RectSpring.img, a spring infrared image with 1 meter cell size, NWILakes, MNDOTLakes, DNRLakes, various renditions of lake boundaries, SouthBayArea, a shapefile for a work area in the second part of this lab. Background: Multi-temporal images are commonly used for updating information, or documenting change. Data often arent current or consistent, and as images get better, we use them to improve our data. Digitizing Tools and Techniques Load the BigMarSum & RectSpring images in a data frame, and also the NWI, MNDOT, and DNR lakes data. Examine each layers; note how the various lakes data differ from each other, and from the lake boundary in the images. While the DNR data matches best, it still has some errors for our intended use, so we need to digitize a new lake boundary before adding vegetation data. You will be digitizing the current boundary only for Big Marine Lake, the biggest lake in the view, based on the RectSpring image. Use the map at the end of this section for guidance, and the examples below that illustrate the land/water boundary for the lake. First, In Arc Catalog, set the path to your Lab 4 data folder, then create a new empty polygon, lake layer (shape file or in a geodatabase) named Lake_2000, (Projected Coordinate SystemsUTMNorth AmericaNAD 1983 (2011) UTM Zone 15N), add it to your data frame, and set display, snapping, and sampling options as appropriate (refer to Lab 3 if youre fuzzy on how to do this). Use a scale of about 1:3,000 or larger (e.g., 1:800) when digitizing in this exercise to record sufficient detail.
When creating large complex polygons, it may be useful to digitize the lake a section at a time, to contain errors and allow frequent saves. One approach (in video last week on polygon digitizing) is to
first digitize a new polygon ( in the Create Features window), and digitize a portion of the lake, and then
add to the polygon with the auto complete tool ( in the Create Features window)
Remember with Auto Complete Polygon your actually create a new polygon adjacent to your original polygon. You will have to merge these separate parts of the lake with the EditMerge (may sure at least two adjacent pieces of the lake are selected).
When auto-completing polygons, you need to be careful to start and end at the shores where the new polygon connects to the old polygon. See the videos on autocomplete polygons in last weeks lab if for help. You can also use a streaming version of auto-completion they call Auto
Complete Freehand ( ) as noted in the videos for Lab 3 which turns on a streaming mode. This means you dont have to laboriously click each vertex, it inserts them automatically. However, you need a steady hand, and can more easily create overlaps, loops, and odd digitizing artifacts, so use with care. Another choice is to add a small polygon and use the Modify Feature Edit tool to add new sections on to your initial polygon (See Video: Merge_AutoComplete_Reshape_Save_Edits) Save your Edits FREQUENTLY. Make sure to digitize the bays, but dont digitize the boat docks as part of the shore. See the example map at the end of this section for guidance. You must digitize islands both in the lake and near shore, as shown in the example map. Perhaps the easiest way to digitize and island is:
First digitize the lake, covering the islands.
Turn the lake fill semi-transparent (properties, then display, set transparency something like 40 50%)
Then digitize a polygon that outlines an island on top of the lake polygon.
Select the island polygon
From the main Editor menu, select the Clip option (see figure)
A window will pop up, make sure the buffer distance is 0 and the discard overlap option is selected
See the Lab 4 video, ClipDigitizing This will clip out the island, but you still have an island polygon. We dont want to include this in our lake layer.
Select the island polygon, right click over it, and then select delete from the window that appears. Also note that there is substantial marshland that is considered part of the lake, and not land, along some sections. You will have to do your best job of identifying the land/water boundary, which is difficult because of similar appearing upland and lake vegetation near shore. Here, well call most wetlands part of the lake, that is, well lump wetland vegetation with the lake. Some wetland grasses grow both in shallow water and upland wet soils, and cattails are rooted but grow out of the water. The RectSpring image is generally much better for identifying the land/water boundary, as much of the aquatic vegetation is dead; there are still dense cattail beds that are part of the lake, but may at first appear like land. Use the examples below to guide you. The two example images illustrate the task. These are of the north shore of the eastern lobe of Big Marine Lake. An approximation of the land/water boundary is shown in yellow. The spring image is shown first. Note the water is dark blue, and cattails/wetland grasses show as dark grey. Roads are also grey, but uniform in color and straight-sided, so shape and texture sets them apart. Upland grasses are white, through pinks to red, so the boundary (in yellow) between upland and lowland is somewhat obvious for most of this stretch. There is a near-shore area of upland, labeled A on the images. This is a bit more complicated and subjective to delineate, because the both the marsh cattails and wetland grasses and the upland grass are shades of grey, so it isnt obvious where the land/water boundary should be. In addition, there are small patches of cattail embedded in the upland, which we include as part of the upland polygon because we cant spend all day digitizing. The summer image (2nd image shown below) helps, because the various shades of light red and pink are upland grass/forbs, while the emergent wetland cattails are blue/black to very dark red. Floating vegetation is also white/pink/light red, but their texture and location (areas that are water in spring, vegetation in summer) gives them away.
Spring Image: Summer Image:
Note that you should be especially careful in digitizing the shoreline in the connected bay at the south end of Big Marine Lake (inside the yellow rectangle, figure to the right). This appears to be a separate lake, but youll see when you zoom in that it is connected to the main body of the lake by a narrow channel. This happens a couple of other places around the lake, and you might notice if you carefully check the example map we provide that we dont always digitize these connected lakes as part of the Big Marine Lake; however, be especially careful in identifying and digitizing the land/water margin in this yellow box. We will use it later in this lab to make topologically consistent layers.
When you are done digitizing the entire lake boundary, create a map that includes the lake polygon you digitized and one of the images as a background (data frame as in the example below). Add a descriptive title, your name, a north arrow and scale bar. Create a pdf document for your map, and turn it in. Lake Boundary to Digitize:
Digitizing into a Geodatabase with Topology We often need to create vector data with topological constrains, e.g., making sure that upland polygons dont overlap with adjacent lake polygons, or that there are no gaps between lakes and uplands, and aquatic vegetation only occurs in lakes. As noted in Lab 1, we can enforce these constraints in a geodatabase, with topology. Well practice creating and digitizing into topology here. First, create a new ArcMap project, and add the lake boundary you digitized in the previous part of this exercise, and the RectSpring and BigSumUTM images. We wish to have four polygon layers in our geodatabase: uplands, lakes and ponds, emergent aquatic vegetation, and floating aquatic vegetation. Create an empty geodatabase. See the end of Lab 1 about topology, and Lab3 for instructions and videos if you are fuzzy on these first few steps on creating a geodatabase. Open ArcCatalog (filing cabinet icon) and right-click within a target directory to add a new geodatabase:
This will open a dropdown list showing a directory tree. You can navigate to a subdirectory where you want to store your new geodatabase. Left click on the subdirectory to select it, then Right click over the directory to open a menu. Select New, then Personal Geodatabase from the menu.
Here I named it BML_Topol. Within the new geodatabase, create a Feature Data Set (right click on the geodatabase, then NEW),
name it something like BML_Layers, and
select the UTM zone 15 (NAD83_2011) horizontal coordinate system, and
use defaults for the rest of your choices. Now to add some data (see Video TopologyRules): right click on the BML_Layers feature data set, then Import: Import