Lab #5: Vector GIS Data Editing and Georeferncing Credit point: 6; Due date: Oct. 18 (Friday 5PM)

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1 Principles of Geographic Information System Geography 390, Fall 2002 Department of Geography, Texas A&M University Instructor: Dr. Hongxing Liu Teaching Assistants: Sindhu George, Jongwon Lee Lab #5: Vector GIS Data Editing and Georeferncing Credit point: 6; Due date: Oct. 18 (Friday 5PM) I. Objectives This lab is a continuation of lab#4. The main purpose is to get students familiar with the process of editing and georeferencing a vector data set digitized from a paper map. Specific tasks include: 1) Editing the vector coverage of the US state boundaries, which was digitized in lab#4; 2) Computationally constructing topological structure for the digitized coverage; and 3) Transforming the coverage coordinates from arbitrary Cartesian coordinate system of digitizing table to a geographical and map coordinate systems. II. Instructions and Exercises 1. Constructing Topological Data Structure ArcInfo uses computational methods to establish the topological relationships between the spatial objects. When the state boundaries were digitized, we did not explicitly encode the topological relationships through digitizing protocol, such as, which arc is directly connected to which arc, which arcs form which polygon, etc. After digitizing, ArcInfo can computationally construct the topology for the coverage. The constructed topological relationships are stored in a set of cross-referenced tables (files). Once the topological relationships are constructed and stored, they can be used for subsequent automated error checking and various vector-based analyses. 1.1 Constructing topology Two commands can be used for constructing topology for a coverage in ArcInfo: clean and build. The command clean creates a new, topologically structured coverage from a spaghetti coverage digitized. This command intersects the digitized arcs and identifies the spatial relationships between nodes, arcs, and polygons. The feature attribute table is created to store the geometric and topological information about the digitized features. The command build is only used to create or update the topology when arcs do not need to be intersected. The coverage states01, which we digitized in last lab exercise, does not have topological structure. It can be converted into a topologically structured coverage states02 using the clean command. Arc: clean states01 states02 # # poly 1

2 A polygon attribute table states02.pat is created for the new coverage states02 to store the geometric and topological information. The content can be shown: Arc: list states02.pat Or Arc: &station 9999 Arc: &popup states02.pat Or Arc: &fullscreen states02.pat 1.2 Identify digitizing errors based on topological data structure Once the topology is constructed, ArcInfo can automatically identify and mark potential node errors with special symbols. This makes the editing process much more efficient. You can also identify most digitizing errors by comparing a plot of the digitized coverage with the original source map. Two kinds of node errors will be marked: pseudo nodes that are labeled with diamond symbols, and dangling nodes that are labeled with box symbols. Pseudo nodes occur where only two arcs intersect or a single line connects with itself (an island). A dangling node is the unconnected node of a dangling arc. It is often created by an undershoot or an overshoot of the arc. It should be noted that pseudo nodes and dangling nodes are not necessarily errors, although they usually are. Label errors in a polygon coverage include two situations: A polygon has no label point, or a polygon has more than one label points. Plot the pseudo nodes and dangling nodes to verify the potential digitizing errors. Arc: editplot states02 states02.gra Window plot? no Enter legend text: <CR> Plot tics? yes Plot Only polygons with label errors? NO Plot arcs? yes Plot arc ids? yes Plot label points? yes Plot node errors? yes Arc: &station 9999 Arc: Draw states02.gra The potential errors will be flagged in the plot. You can zoom in and out to get a close-up look at the errors. Print out a hardcopy of states02.gra using the print program in the Arc/Info program group. Generate a list of potential errors on the screen, and use a watch file to store the information in a text file. For node errors: Arc: &watch node.err Arc: nodeerrors states02 all Arc: &watch &off For label errors: Arc: &watch label.err 2

3 Arc: labelerrors states02 Arc: &watch &off To check if the two files have been created Arc: &sys dir /w To view the file contents: Arc: &station 9999 Arc: &popup label.err Arc: &popup node.err Open label.err and node.err using Notepad and print out these two files as a part of your lab report. 2. Editing the Errors in the Coverage Once the errors are identified, you can verify and correct all the potential errors on screen with the aid of ArcEdit module. 1) Set editing environment For safety sake, make a copy of the coverage to be edited, and then specify this coverage as the editing coverage. Arc: copy states02 states03 Set the editing environment Arc: arcedit Arcedit: &station 9999 Arcedit: coordinate mouse Arcedit: edit states03 Arcedit: drawenvironment arc node label Arcedit: draw Arcedit: nodecolor node red Arcedit: draw Arcedit: drawenvironment node errors label off Arcedit: draw Specifying the editing feature class (tics, arcs, nodes, labels, or any other feature classes); Arcedit: editfeature arc 2) Select the features in errors You can use pan/zoom button at the top left corner of your display to zoom in, zoom out, and roam your coverage. Once you locate the features to be edited, you can issue command to select these features. Arcedit: select many 3

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5 The cursor will appear on the screen. Position the crosshairs directly over the feature you wish to edit, and press the left button on the mouse or the 1 button on keyboard to select it. Press 9 on keyboard to quit from the selection mode. 3) Editing the features Arcs can be deleted, moved, added, or copied using the delete, move, add, or copy command. If you make a mistake in deleting a feature, you can use oops to restore it. Labels can also be selected and edited. To edit label points, the current edit feature has to be set to label. Nodes can be deleted, added, or copied only if the arc they are attached to is deleted, added, or copied. Move command can be directly apply on node 5

6 feature class. To avoid accidental loss of your editing work, remember to save your work regularly. Arcedit: save Usually we correct an overshoot error by deleting the overshoot arc, correct an undershoot error by extending it, and fix an open polygon error by moving the nodes to close the polygon. The preceding figures shows the commands used to add a missing label point, add a missing arc, correct an overshoot, correct an undershoot, fix an open polygon, and change a label User-ID. 4) Quit the Arcedit session Once you have done with all the editing work, you can save your work and quit from ArcEdit. Arcedit: quit Keep all edit changes (Y/N): y Leaving ARCEDIT 3. Reconstruct Topology After editing the coverage, the topological relationships between spatial objects are changed. We have to use the command clean to reconstruct the topology. Remember that you have to run clean again to reconstruct the coverage topology when you complete the editing work with arcedit. Arc: clean states03 states04 Once you establish the topology for the new coverage, you need to start the second round of error checking and see if all errors have been fixed. Arc: nodeerrors states04 all Arc: labelerrors states04 If some errors remain, you have to conduct another arcedit session and repeat all steps above until all error are corrected or removed. 4. Transforming Cartesian Coordinates to Geographic Coordinates At this point, the coordinate system of TIC point coverage usa_tic and the polygon coverage states04 is in inches, referenced to an arbitrary Cartesian coordinate system of the digitizing table. Since we know the geographic coordinates of TIC points in latitude and longitude and map projection, we can use TIC points as control points to register the polygon coverage states05 to geographic coordinates. Make a table that contains the Tic-ID, longitudes and latitudes in decimal degrees of all TIC points by reading the graticule of the base map. Make sure that longitude goes before the latitude. If the Tic locations are recorded in the projected map coordinates, the transformation process directly converts the existing digitizer measurements to real-world coordinates. If, as in our case, the Tic locations are recorded in latitude and longitude, the TIC coverage needs to be projected into the map projection of the base map before performing the transformation of the state boundary polygon coverage. 1) Create a TIC coverage for editing Arc: create geo_ref states04 6

7 With the above command, a new empty coverage geo_ref is created. This coverage does not contain any spatial objects and only contains the same set of TIC points as the polygon coverage states04. The coordinates of the TIC points can be displayed. Arc: list geo_ref.tic Arc: list states04.tic We will edit geo_ref.tic for converting the coordinates of the coverage into real world geographic coordinates. 2) Update TIC coordinates into longitude-latitude Coordinates The info command update will be used to change the xtic and ytic value from the digitizer units (inches) to real world geographic coordinates. To use info appropriately, you need to type every command in upper case. You can press Caps Lock on keyboard and then type info commands. Start info module Arc: info Enter user name>arc Enter command>select GEO_REF.TIC Enter command>update PROMPT Info will show $RECNO> prompt. Specify the record that you want to edit by typing in the record number. $RECNO> 1<CR> Info will give IDTIC, XTIC and YTIC values of the first TIC point in geo_ref.tic file, and then ask if you want to make change to the id and xtic and ytic values. We do not need to change tic-id value, just press Enter button on keyboard in response to IDTIC prompt. IDTIC> <CR> For XTIC> and YTIC> prompts, type in their corresponding longitude and latitude values in decimal degrees. IMPORTANT: remember to use longitude value for XTIC, and latitude value for YTIC. After the first TIC point is done, info will prompt you for the next. Repeat the updating process for the rest of the TIC points. When all the TIC points are done, press the Enter key on the keyboard to respond to the $RECNO> prompt. $RECNO> <CR> This ends the updating mode. Now, you can use list to show the updates. Enter command> list When you complete the updating of the coordinates of TIC points, you can quit from info: ENTER COMMAND> Q STOP. 3) Project TIC coverage from Geographic to Albers Equal-Area Conic Projection. Because the original base map that you digitized is in the Albers Equal-Area conic projection, you need to project the Tic coordinates recorded in decimal degrees of latitude and longitude into the Albers coordinates, which will form the basis for the real-world coordinates onto which the state boundary coverage will ultimately be transformed. 7

8 You can use the projection file geo_albers.prj edited in the last lab to do it. Arc: project cover geo_ref albers_ref geo_albers.prj Recall that the content of geo_albers.prj is: INPUT PROJECTION GEOGRAPHIC UNITS DD PARAMETERS OUTPUT PROJECTION ALBERS UNITS METERS PARAMETERS END Arc: list albers_ref.tic The coordinates of TIC points in the coverage albers_ref.tic have been automatically changed into the Albers Equal-area Conic projection coordinates. 3) Project the state boundary coverage into Albers Equal-area Conic projection The Arc command transform converts coverage coordinates using either an affine or a projective transformation. The affine transformation function calculates changes in scale, shift in the x-direction, shift in y-direction, and any rotation for the output coverage. Before using transform, create an output coverage containing only the projected tics: Arc: create states05 albers_ref The command transform compares corresponding TIC points between the input coverage and the output coverage and then derives a mathematical formula to perform transformation of spatial objects in the coverage. Arc: transform states04 states05 A report will be generated to show the RMSE of the transformation. Take a look at the coordinate information of your final coverage states05. Arc: describe states05 5. Understanding Topological Structures As stated earlier, ArcInfo uses a computational method to construct the explicit topological relationships between spatial objects (nodes, arcs, and polygons). Commands clean and build are used to perform topology construction in ArcInfo. The topological information is recorded and stored in a set of data files in a tabular format. During the topology construction, a Polygon Attribute Table (PAT) is created for a polygon coverage, an Arc Attribute Table (AAT) for a line coverage, and a Point Attribute Table (PAT) for a point coverage. Some information about the geometry and topology is hidden from the end users, although the information is stored in some data files. To show the Polygon Attribute Table for states05, use the following commands: Arc: items states05.pat 8

9 Arc: list states05.pat Note that one polygon (states05# = 1) has a negative area value. BUILD with the POLY option creates one additional polygon called the background polygon. This is a special polygon referred to as universe (external, or background) polygon. It represents the area beyond the outer boundary of the coverage. It is always given polygon internal number 1, namely, states05# = 1, and its area is the total sum of the areas of all other polygons in the coverage. It is shown as a negative AREA in the PAT. This polygon makes every arc have a left and right polygon. To look at the topological relationships between nodes, arcs, and polygons, you need to do the following: Arc: clean states05 states05 # # line Arc: items states05.aat Arc: list states05.aat You can also extract the node points from the polygon coverage states05, and take a look at the Point Attribute Table of the node coverage: Arc: renode states05 Arc: nodepoint states05 nodes05 Arc: build nodes05 point Arc: addxy nodes05 point Arc: items nodes05.pat Arc: list nodes05.pat To look at the coordinates (geometry) of nodes, arcs, and polygons, do the following: Arc: ungenerate point nodes05 nodes05.txt Arc: ungenerate line states05 arcs05.txt Arc: build states05 poly Arc: ungenerate poly states05 polys05.txt Use Nodepad to open nodes05.txt, arcs05.txt, and polys05.txt. Observe the data structure, and geometry information in these files. 6. Integrating the State Boundary Coverage with other GIS data The polygon coverage states05 has a topological structure and is in a correct map projection. Now, you can integrate this coverage with other data to do spatial analysis. Here, we will only perform a simple visual overlay between the state boundary polygon and a digital elevation model of the US: 1) Start ArcView, load a digital elevation model of the US from W:\lab05\usdem. To load this theme, you have to click File->Extensions, then check 3D analyst. In the add theme window, select Grid Data Sources to find usdem. 2) Change the default legend of usdem by double clicking the theme usdem and display it using continuous gray shade. 3) Load the state polygon coverage you just digitized and edited into the view window. Set the state boundaries color as red without filling colors inside. 4) Load the state boundary coverage states_a from W:\lab05\states_a, which is available in Arc Data provided by ESRI. Set the state boundaries color as yellow without filling colors inside. 5) Annotate your home state with the state s name. This can be done using the text label tool ( T icon). 9

10 6) Make a layout using the view. Add and edit the map title, scale bar, north arrow, and your name on the map. Customize each of these cartographic elements and make them aesthetically pleasing. 7) Make a hard copy of this composite map. III. Lab Report The lab report must be typed. The lab report should complete the tasks and answer the questions listed below: 1. Comparing the state boundary states05 that you digitized and the state boundary coverage states_a provided by ESRI in Arc Data, which parts of your coverage are obviously less accurate than that provided by ESRI? 2. We updated the coordinates of the TIC points in geo_ref from the arbitrary coordinates of the digitizer (in inches) to the geographic coordinates in longitude and latitude in decimal degree. Then, we projected the TIC coverage geo_ref to albers_ref in Albers Equal-area conic projection before transforming the coverage state04 to the real-world coordinates of the coverage states05. Why do we need to project the TIC coverage geo_ref from geographic coordinates into the Albers Equalarea conic map projection? 3. Why do we need to construct the topology before we identify the potential digitizing errors? Why do we need to reconstruct the topology after the editing? 4. What types of errors did you identify and correct during the editing process? Please include a hardcopy of states02.grd to illustrate it. Also include a hardcopy of node.err and label.err. 5. What items are in states05.pat, states05.aat, and nodes05.pat? How are these items related to the geometric and topological properties of spatial objects in the coverages? 6. Describe the data structures of the ungenerated files nodes05.txt, arcs05.txt, and poly05.txt. Include a small portion of each file to illustrate your description (Hints: Use the on-line help to look at the explanations on ungenerate command.) 7. Why do you think the coverage states05 that we digitized and edited can integrate with the grid usdem and the coverage states_a provided by ESRI to do overlay analysis? Include a hard copy of the composite map produced in section 6. 10