Introduction to GIS 2011

Transcription

1 Introduction to GIS 2011 Digital Elevation Models CREATING A TIN SURFACE FROM CONTOUR LINES 1. Start ArcCatalog from either Desktop or Start Menu. 2. In ArcCatalog, create a new folder dem under your c:\introgis_ Copy the data from g:\intogis_2011_main\dem into the folder on the C drive. 4. There are 4 Zipped files folders containing contour data. Extract them all on your C drive. 5. Using the clip tool from ArcToolbox (Analysis extract clip) Clip the contour data with the UNBC_outline.shp Create four layers PG_Cont1, PG_Cont5 Clip the Hydline.shp and Hydpoly.shp with the UNBC Outline as well. Call them Hydline_cl.shp and hydpoly_cl.shp ***Note that the clipping layer is square TO WORK ON SURFACE MODELING, YOU NEED ARCGIS 3D ANALYST EXTENSION ENABLED IN ARCMAP. 1. Start ArcMap with a new empty ArcMap file. 2. Add datasets PG_Cont1, PG_Cont5, PG_Cont10, PG_Cont20, Hydpoly_cl.shp and Hydline_cl.shp to from your C drive 3. In ArcMap, select Tools->Extension and make sure the check box beside 3D Analyst is checked. Close the window. 4. Turn on 3D Analyst tool bar by clicking on View->Toolbars->3D Analyst. You should see the 3D Analyst toolbar pop up. SET UP THE WORKING DIRECTORY. 1. Click 3D Analyst -> Options. Under the General tab, set the working directory to C:\IntroGIS_2011\DEM. This will cause all outputs to be saved under this directory. 2. Use the Identify tool to examine the contour lines. The elevation value is stored in the data field ELEVATION Click some other contour lines to see their elevation.? What is the elevation interval for the 4 contour layers

2 CREATING A TIN 1. Click on the 3D Analyst on the toolbar and choose Create->Modify TIN->Create TIN From Features. We are going to use all four data sets to create a four TIN surface. You can use more datasets as the input data to create TIN such as break lines, Lake Boundary etc to get better TIN surface. The more information you add into the input data will result better TIN surface. * Note at a later time we will play with the other settings. 2. Check PG_Cont1 in the Layers list, choose ELEVATION from dropdown list for Height Source, and mass points for Triangulated as. 3. Set the Output TIN to C:\IntroGIS_2011\DEM. This will case the output to be saved under your dem directory. 4. Call the new TIN PG_Cont1_TIN 5. Click OK to build a TIN surface. It might take awhile. 6. Once the operation is finished, you will have a TIN surface added in ArcMap. Be patient, it takes a while to load TIN. You will see a result similar to the image right above. 7. Follow the same process for all 4 layers

3 Compare the 4 resulting layers and answer these questions? What are the differences?? Were the results what you expected?? What are the 2 highest elevation points on the PG_Cont5_TIN?

4 VIEWING TIN SURFACES IN ARCMAP TINs are made up of triangular facets. The nodes and edges are the elements that make up the triangles. They may also contain break lines lines that follow sets of edges that play important roles in defining the shape of surface (roads, rivers). You can display one type of TIN feature in a map or scene for example, just the triangles or all of the TIN features. You can also symbolize each type of feature in different ways. DRAWING TIN FACES BY ELEVATION 1. In the table of contents, right-click the PGcont1_tin and choose Properties. 2. Click the Symbology tab. 3. Click Elevation if it is not checked 4. Optionally, click the Color Ramp dropdown list and click a new color ramp. 5. Optionally, click the Classes dropdown list and choose the number of classes. 6. Click Apply. DRAWING TIN FACES BY ASPECT 1. In the Layer Properties window, uncheck Elevation (or any other method that is checked). 2. Click the Add button. This will pop up the Add Renderer window which allows you to choose how to view the TIN data. 3. A list of renderings is listed at left. Click the first one 'Edge type grouped with unique symbol'. The overview is displayed at right. 4. Click Add to add this into the Show Box in the Layer Properties window. 5. Repeat the steps to add the rest except 'Face tag value grouped with unique symbol' and 'Node tag value grouped with unique symbol'. 6. Click Dismiss to close the Add Renderer window. 7. Now back to the Layer Properties window for cont5_tin. You should see that more Renderer methods are now listed in the Show box.

5 8. Click the check box beside Aspect and uncheck all the rest. Click Apply to see the result. This will show the TIN in Face aspect with graduated color ramp. The TIN faces are rendered with colors to indicate the direction they face to. DRAWING TIN FACES BY SLOPE 1. In the similar way, view the TIN by slope. 2. Uncheck all renderers and turn on Slope in the Show box. 3. Click Apply. The TIN faces are rendered with colors to indicate the slope of the terrain. Feel free to play around with other methods listed in the Show box to see the changes. YOU CAN ALSO DISPLAY THE TIN BY COMBINING DIFFERENT RENDERING METHODS. 1. Use the Up and Down arrow keys beside the Add buttons to change the order of items in the Show box. 2. Click Nodes in the Show box and click the Up arrow key to move it to the top of the list. 3. Rearrange the order of the items in the list to make them show as the image at left below. This will cause the most layers to be visible in the display window. 4. Turn on Nodes, Edges, Elevation and turn off all the rest. This will cause the nodes, edges of the triangles and the filled faces with graduated color to be displayed in the window. 5. Use Zoom In tool to zoom in to examine the data. You should see the nodes with edge lines of triangles and the faces. 6. Try other combinations to see the changes.

6 SECTION 2 The first portion of this lab had you clipping data using a Square. Let s try clipping the contour data using a different shaped polygon. 1. Add your PG_Cont5 layer and the UNBC_OutlineNS.shp layer to the map. 2. In Arctoolbox, clip your PG_cont5 data with the UNBC_OutlineNS.shp. 3. Follow the steps that are required to create the new TIN data set. 4. Note the difference in the data in the North-west corner. The data extends from each edge of the clipped corner. Although it shows data, the information has been incorrectly interpolated by the application. The resulting output is a HULL (see below from ESRI 9.2 desktop help) Hull The hull of a TIN is formed by one or more polygons containing the entire set of data points used to construct the TIN. The hull polygons define the zone of interpolation of the TIN. Inside or on the edge of the hull polygons, it is possible to interpolate surface z-values, perform analysis, and generate surface displays. Outside the hull polygons, it is not possible to derive information about the surface. The hull of a TIN can be formed by one or more polygons, which can be non-convex. A non-convex hull must be user-defined by including Clip and Erase exclusion features during the construction of the TIN. These features explicitly define the edge of the surface. When no exclusion features are used to define the hull, the TIN generator creates a convex hull to define the bounding edges of the TIN. A convex hull is a polygon with the property that any line connecting any two points of the TIN must itself lie inside or define the edge of the convex hull. The definition of a non-convex hull is essential to prevent the generation of erroneous information in regions of the TIN outside the actual dataset but inside the convex hull. Consider the diagram below. The way to resolve this is to add hard and soft break lines. These are lines which will define changes in aspect, slope as well as delineate project areas. Below are the ESRI summary definitions of the hard and soft break lines and Hulls

7 Hard break lines Hard break lines represent a discontinuity in the slope of the surface. Streams and road cuts could be included in a TIN as hard break lines. Hard break lines capture abrupt changes in a surface and improve the display and analysis of TINs. Soft break lines Soft break lines allow you to add edges to a TIN to capture linear features that do not alter the local slope of a surface. Study area boundaries could be included in a TIN as soft break lines to capture their position without affecting the shape of the surface. Hulls Polygons represent surface features with area such as lakes or boundaries (also called hulls) of separately interpolated areas. Take you newly clipped 5m data and let s generate a new tin, this time without the Hull. This can be done one of 2 ways; Creating a new TIN or modifying an existing TIN Creating a new TIN 1. In the 3D Analyst tool bar go to create TIN from features. 2. Selected your clipped contour file as the layer you will be creating the TIN from. 3. Select the UNBC_OutlineNS layer and set the height source to <None> and Triangulate As to soft clip (see graphic). 4. Set your output file and select OK? What was the final result? Editing and existing TIN 1. In the 3D Analyst tool bar go to Add features to TIN. 2. Select your input TIN. 3. Select the UNBC_OutlineNS layer and set the height source to <None> and Triangulate As to soft clip (see graphic). 4. Set your output file and select OK 5. The result is the same as the previous process but you didn t have to create a new TIN

8 QUERYING SURFACE DATA Sometimes just looking at 3D data is not enough. We often need to query data or derive new data from the surface TIN to solve problems. 3D Analyst lets you explore the data on a map or in a 3D scene and get the information you need. Use Identify tool (i button) to query the TIN surface. When you click to on a TIN surface, you will get elevation, slope, and aspect of the location where you clicked. Identifying features Click the Identify button on the Tools toolbar (in ArcMap) or on the 3D View Tools toolbar (in ArcCatalog). Click the mouse pointer over the feature (PG_cont5_tin) you want to identify in the display window. The Identify window pops up showing the elevation, slope, and aspect value of the location where you clicked. Note: make sure the layer in the popped up window is set to PG_cont5_tin Displaying map tips with elevation value for surfaces In the ArcMap table of contents, right-click the PG_cont5_tin and click Properties. Click the Display tab. Check Show Map Tips. Click Fields and choose Elevation for Primary Display Field from the dropdown list. Click OK. Move the mouse pointer over a TIN facet. You should see the elevation value showing up. Now add the dataset lookouts from G:\IntroGIS_2011_Main 2a) What is the elevation for each lookout location? 2b) Which lookout has higher elevation (western or eastern)? 3a) What is the aspect for each lookout location? 3b) Which direction does each lookout face to in compass directions? 4a) What is meant by slope having zero value and 4b)What is meant by aspect having negative value?

9 MANAGING 3D DATA IN ARCCATALOG Previewing and navigating 3D data ArcCatalog lets you manage and preview your 3D data. In ArcCatalog, when the selected item in the Catalog tree contains geographic data, you can preview the data without having to create a map or 3D scene. TINs and feature classes with z-values embedded in their geometry for example, PointZ, PolyLineZ, PolygonZ, and Multipatch shapes will be automatically rendered in 3D when you select 3D View. Raster and 2D feature classes will be rendered as though they rest on a flat surface. You can create layers that specify 3D rendering properties for all types of data, and the layers will be rendered in 3D. Minimize ArcMap window and back to ArcCatalog. Make sure that you have the 3D Analyst extension enabled in ArcCatalog. Click on Tools- >Extension and check the mark for 3D extension if it is not on. View->Toolbars->3D View Tools to make the toolbar display on the screen. The 3D View Tools let you navigate around your data in 3D. Some of these tools have same functionality as you saw in ArcMap and ArcCatalog. Other tools are specific for 3D perspective viewing. The Navigate tool combines the functions of several of other tools. Clicking the left, right, and center mouse buttons, and dragging up, down, left, or right, these buttons let you rotate the 3D view, zoom in and out, and pan across the view. In the catalog tree in ArcCatalog, navigate to your local folder dem and click the TIN you just created (PG_cont5_tin). Click the Preview tab. Click Preview dropdown arrow (located at lower part of the display window) and chose 3D View. The buttons on the 3D toolbar are enabled. Click the Navigate button. The Navigate button allows you to rotate the view in 3D. Click the display area in the display window and drag it to the right to rotate the view counter clockwise around the z-axis. Right-click on the display area and drag down to zoom in to the data. Try to drag up to see what happens. Hold on both mouse buttons or the center button on a three-button mouse) and drag the data to the right to pan the view. Click the display area and drag up. That will lower your viewing location relative to the data. Click the Full Extent button (the global button). The data returns to the original extent and view position. Now create a thumbnail for the 3D view by clicking on the second button from the right on 3D View toolbar.

10 CONVERTING A TIN SURFACE TO RASTER The TIN can be converted to raster surface data. 1. Now back to ArcMap window. On the 3D Analyst toolbar, click the 3D Analyst - >Convert->TIN To Raster. 2. Set PG_cont5_tin for Input TIN 3. Set Slope (in degrees) from the dropdown arrow for Attribute. This will convert slope information in degree to raster data. You can choose Elevation, Aspect, Slope in percentage for other information conversion. Optionally, set a z-factor. The z-factor is used to convert the z units to the same scale as the x and y units if they are different. We just use the default. Set 5 for cell size of the output raster. Set Output raster C:\introGIS_2011\DEM\Tin_Slope Click OK. The output dataset will be added to ArcMap automatically with default symbology (could be gray scale). You can change the symbology from the Layers Properties window. The slope raster can be used to find out the slope information at a location. Slope converted from TIN (tin_slope) Raster DEM converted from TIN (tin_elev) In the same way, convert the TIN (PG_cont5_tin) to raster elevation. Choose Elevation for Attribute and C:\introGIS_2011\DEM\tin_elev for Output raster. Keep the rest parameters same as you did for the tin_slope raster.