Geodatabase and Topology

Exercise 1: Preparation of Digital Topographic Database: Database Preparation and Coordinate Definition

Introduction Geodatabase The Geodatabase is the common data storage and management framework for ArcGIS. It combines "geo" (spatial data) with "database" (data storehouse) to create a central data repository for spatial data storage and management.

The Geodatabase offers you the ability to:
➢ Store a rich collection of spatial data in a centralized location. ➢ Apply sophisticated rules and relationships to the data. ➢ Define advanced geospatial relational models (e.g., topologies, networks). ➢ Work within a multiuser access and editing environment. ➢ Integrate spatial data with other IT databases.

Coordinate Definition Raster data is commonly obtained by scanning maps or collecting aerial photographs and satellite images. Scanned map datasets don't normally contain spatial reference information. With aerial photography and satellite imagery, sometimes the location information delivered with them is inadequate and the data does not align properly with other data you have. Thus, to use some raster datasets in conjunction with your other spatial data, you may need to align, or georeference, to a map coordinate system. Geo-referencing is the process of aligning a raster data set to known map coordinates and assigning a coordinate system. Geo-referencing creates additional information within the file itself and/or in supplementary files that accompany the image file that tells GIS software how to properly place and draw it. Objectives ➢ To create a Geodatabase ➢ To create GIS layers (Point, Line, and Polygon) ➢ To define coordinate system of a raster data (Scanned Image )

1.1 Creating a Geodatabase

In this section, you will learn to create a geodatabse using ArcCatalog. This application can be used to organize and manage various types of geographic information for ArcGIS such as geodatabases, raster files, geoprocessing toolboxex, etc.

1. Open ArcCatalog You can open it by two processes (Either Start>All Programs> ArcGIS> ArcCatalog10 or Open ArcGIS and click catalog under windows button on the main menu.

2. On the right side of the ArcCatalog window, right click on the folder where you wish to create a Geodatabase.

3. Go to New and Click Personal Geodatabase. (New Geodatabase is created with the default name New Personal Geodatabase)

4. To rename it, right click and click Rename.

5. Delete the existing name and rename to Damauli.mdb

You have now successfully created a personal Geodatabase. You will now create GIS layers within it. 1.2 Creating GIS Layers

In this section you will create vector layers (Point, Line, and Polygon).
a. Creating Point Feature

1. Right click on Damauli.mdb , go to New and click New Feature Class
2. A dialog box, New Feature Class appears. Name the feature class as SpotHeight. From drop down arrow, change Type of features to Point Features and click Next.

Now you have to define the coordinate system of the feature. For this, you will import the coordinate system of existing layer.

3. In the New Feature Class dialog box, click Import.

4. Navigate to D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Data and select Building.shp and click Add.

5. Click Next and Finish to complete the process. b. Creating Line Feature

6. Right click on Damauli.mdb , go to New and click New Feature Class.

7. Name the feature class as Road. From drop down arrow, change Type of features to Line Features and click Next.

8. Repeat steps 3-5 of above (Creating Point) to complete the process. c. Creating Polygon Feature

9. Right click on Damauli.mdb , go to New and click New Feature Class.

10. Name the feature class as LandCover. From drop down arrow, change Type of features to Polygon Features and click Next.

11. Repeat steps 3-5 of above (Creating Point) to complete the process.

1.3 Geo-referencing a raster dataset

In this section, you we will geo-reference a scanned topographic map using Arc map. To geo- reference we need ground control points. Here, we will use the map coordinates based on the grid lines.

1. Start ArcMap.

2. Click button from toolbar and add raster data “image11.img” in Arc Map (D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Data).

3. Define Map units and Display units to Meters in Data Frame Properties dialog box.

4. Add the Geo-referencing toolbar if it is not already present, by selecting: Customize> Toolbars > Geo-referencing

5. From the Geo-referencing toolbar, click the Layer drop-down arrow and click the raster layer you want to geo-reference i.e. “image11.img”.

6. Pick at least four well distributed coordinates (x, y) from your raster data i.e. “image11.img” (Hint: use grid lines to identify x, y position of the point. It is better to use grid intersection)

The coordinate for first grid intersection is for this is (525000, 3097000). The coordinates of other grid intersections can then be determined. 7. Click the Add Control Points tool to add control points. [It is a good idea to zoom in on your image when adding control points for better accuracy] 8. To add a control point, click on a known location on the raster dataset and then right click and choose input X and Y ……

9. Add enough control points (minimum four points) - repeat steps 7-8. [It is better to use maximum number of well distributed control points to improve your accuracy]

10. Open link table by clicking on link table icon

11. Observe the RMSE Error & residuals of each point. If you’re satisfied with the registration, you can stop entering control points.

12. You can remove the point having higher residual value to minimize your RMSE error and add more control points

13. View Link Table to evaluate the transformation. [You need a minimum of three points for a spline or 1st-order polynomial (affine), six control points for a 2nd-order polynomial, and ten control points for an affine or 3rd-order polynomial You can examine the residual error for each link and the RMS error ]

14. You can permanently transform your raster after geo-referencing by using the Rectify command. Click Geo-referencing and click Rectify for this.

15. Save rectified image with file extension .img or .tiff.

You have now georeferenced the image but its projection system is still undefined. To define the projection:

1. Activate ArcToolbox by clicking on if it is not activated.

2. Go to Data management Tools > Projection and Transformations > Define Projection.

3. The input dataset is the rectified image. To define the coordinate system, import the projection system as follows:

4. Click Import and add “Building.shp” layer from D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Data.

5. Click OK. You have successfully completed the first exercise.

Exercise 2: Spatial Data Preparation and Editing

Introduction Geospatial data is the data or information that identifies the geographic location of features and boundaries on Earth, such as natural or constructed features, and more. Spatial data is usually stored as coordinates and topology, and is data that can be mapped. Spatial data is often accessed, manipulated or analyzed through Geographic Information Systems (GIS). Digitizing is the process of converting analog information into a digital representation. In regards to spatial information one application of this is the process of creating a vector digital database by creating point, line and polygon objects. Some basic issues related to digitizing: ➢ Digitizing point features is simply the process of creating a single point feature with an x,y coordinate ➢ Digitizing lines involves creating a line feature that consists of a node (start and end) and a series of vertices which indicate a change of direction along that line. Straight features require fewer vertices; curved/complex features require more vertices. ➢ Digitizing Polygons involves creating a set of connected lines. Editing tools simplify the process of closing polygon features Objectives ➢ To create all the GIS layers required for digitization. ➢ To digitize the features from the rectified image (from exercise 1). ➢ To add fields in the attribute table ➢ To enter the data in the fields of attribute table.

In the scanned topo map, you can see different details like building points, roads, rivers, etc. In order to represent all these features spatially, you have to digitize them in suitable scale and form (like buildings in points, roads in lines and rivers in polygon). In this exercise, you will learn to prepare the spatial data by digitizing the features from image. Also, you will learn to add fields in the attribute table for point features and fill their respective values in the table. 2.1 Add field in Attribute Table of Point Feature In this section you will digitize point features like building, spot height, village names, etc. To digitize:

1. Open ArcMap.

2. Add a point shapefile, “Spot Height.shp” from D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Damauli.mdb.
   (You have created this shapefile in exercise 1)

3. Right click “Spot Height” layer and click Open Attribute Table.

4. From Table options click Add Field.

5. Name the field as Value, select Type as Long Integer and click OK

2.2 Digitize Point Feature

1. Add the image that you have georeferenced in previous exercise.

2. Add the Editor Toolbar (Right click on main menu and select Editor).

3. Click the Editor menu and click Start Editing. The Editor Toolbar is now active. (If your map contains editable data from more than one folder, choose the collection that you want to edit.)

4. Activate Create Features Window (Editor> Editing Windows> Create Features).

5. Select the target spot height & select point tool from the Construction tool palette.

6. Now, zoom in the image and add points (Spot height) by clicking on the topo map.

7. After you click a point on spot height, fill the respective elevation values in the attribute table.

8. Save Edits: When you are in the middle of the editing session, Press Save edits in the Editor Menu.

9. Stop Editing: Once all the required point featured are digitized, Press Stop editing in the Editor Menu.

10. Similarly, you can digitize the points for spot height and add values in the attribute table. IN this way you can digitize other point features such as buildings and village names. 2.3 Digitizing Line Feature You will now digitize the linear features like Roads, River lines, Electric lines etc as you have digitized point features in previous section.

11. Add a line shapefile, “Roads.shp” from D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Damauli.mdb.

12. Start editing process, select target Roads and be sure that the editing options appear in construction palette.

13. Digitize the roads.

14. Save the edits in between and stop the editing process after you complete the process.

2.4 Digitizing Polygon feature Here, you will digitize polygon features like Ponds, River, Forest etc like we digitized point/line features in previous section.

1. Add a polygon shapefile, “LandCover.shp” from D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Damauli.mdb.

2. Start editing process, select target LandCover and be sure that the editing options appear in construction palette.

3. Digitize different land cover in polygons. (Hint: Use Cut polygon Tool from Editor Toolbar to digitize the land cover and digitize the big features first)

4. Save the edits in between and stop the editing process after you complete the process. Similarly, you can digitize all the features and prepare a complete database of the scanned image. You can visualize these features in map.

Exercise 3: Spatial Data Relationship and Topology

Introduction A geodatabase topology defines a structured set of permissible spatial relationships between features within a feature class or between features in two feature classes. These relationships are specified by topology rules when the topology is created. The Topology toolbar provides tools to help you find and correct topology errors and also provides editing tools to help you avoid creating topology errors when editing features in a topology. In geodatabase, topology is the arrangement that defines how point, line, and polygon features share coincident geometry. For example, street centerlines and census blocks share common geometry and adjacent soil polygons share their common boundaries.
• Creating Geodatabase Topology involves
– Establishing the cluster tolerance
– Setting Ranks
– Defining Topology Rules
• Validation of the Topology
• Managing Errors and Exceptions

Objectives ➢ To create geodatabase topology ➢ To edit and validate topology ➢ To provide attributes to spatial data

3.1 Creating a Geodatabase Topology In this section, you will learn how to create a geodatabase topology using ArcCatalog.

1. Start ArcCatalog.

2. Navigate to Damauli.mdb, right click on it and point to New and select Feature Dataset.

3. Type name: Topology and press Next.

4. Import the coordinate system from D:\Advanced GIS Training\Ex1- Georeferencing\Data\Building.shp.

5. Right click on Topology, click on Import and press Feature class (Single).

6. Input feature class: Landcover from D:\Advanced GIS Training\Ex1- DB Preparation & Georeferencing\Damauli.mdb.

7. Fill up other information and click OK.

8. Right click Topology feature dataset, point to New and press Topology.

9. Topology dialog box appears.

10. Press Next then enter name of your Topology: Landcover_Topology and set a cluster tolerance of 0.1 meters.

11. Press Next and then Select All.

12. Press Next and set rank 1. (We don’t want to move our layers.)

13. Press Next and click on Add Rule. Now you have to add the rules which will be used to check the topology of the feature class.

14. On the Add Rule dialog box, select Landcover in Features of feature class and Rule: Must not overlap.

15. Press OK.

16. Similarly add another Rule: Most not have Gaps.

17. After adding all the rules, press Next and you will see the summary as shown below:

18. Press Finish.

19. Now you will receive a message asking for validation, press Yes.

You have created Landcover_Topology to validate errors like overlap or gap in landcover patches. You will further work with road layer. For road layer, repeat all above processes by replacing Landcover with road data.

3.2 Editing and Validating Topology
In this section, you will edit and validate the previously created topology.

1. Open ArcMap, add Landcover feature data set. We will now observe the errors from topology rules and try to edit those errors.

2. Add Topology toolbar from Editor > More editing tools.

3. Click on Validate entire topology.

4. Now click on Topology Error Inspector.

5. In the Inspector window, select one of the rule types and see the error highlighted in ArcMap Window.

6. Now move to map display and try to edit by using different editing tools.

7. Repeat process 3 – 7 to correct all the errors.

8. After all the topology errors are corrected and validated, Save and Stop Editing. Follow the similar process to edit and validate the topology of the roads layer.

   3.3 Attribute Data Entry All the features that you have digitized have a unique feature code. So, to fill up these codes follow these procedures for each layer:

9. Add field named FCode.

10. Start the editing process and provide the codes as given in the table below: Feature Class Feature Code Building 15101 Spot Height 20400 Road 10141 Contour 20110 Forest 25212 Grass 25252 Sand 25332 River 30131 Pond 30402

In case of polygon features, you can also calculate their area. For this:

3. Add a field named Area1 in Landcover feature.
4. Right click in the field Area1 and click Calculate geometry.
5. Select Area in the property bar & Square [m] in units and click OK.

Area of each polygon in the layer is calculated in square meters. You have successfully completed this exercise.