Creating Spatial data

Credits: The original version of this lab module was created by graduate students at UC Santa Barbara, Department of Geography © 2000-2005 Regents of the University of California. Used with permission.

You have been assigned to digitize a new subdivision being planned in your city.  You have been provided with a scanned paper map of the platted parcels and a file with owner information, and will also be estimating how utilities will serve the properties.

Creating a workspace

Make a new file geodatabase

  1. In ArcMap, open the Catalog pane (Windows > Catalog).
  2. Right-click the lab folder you created for this activity and choose New > File Geodatabase. Name it City.
  3. Right-click on the City geodatabase and select Make Default Geodatabase.

Define feature datasets and feature classes

You first need to create feature datasets to hold the feature classes (layers) you will digitize.

  1. Right-click on the empty geodatabase and choose New > Feature Dataset (NOT Feature Class). In the dialog box that opens, specify the name as Utilities – we’ll create a feature dataset called “Utilities”. Click Next to choose the coordinate system. Navigate to Projected Coordinate Systems > State Plane> NAD 1927 and select California Zone V (NAD 1927 StatePlane California Central FIPS 0405).
  2. Click Next, skip the vertical coordinate system by clicking Next again. Accept the default tolerances by clicking Finish.
  3. Right-click on the Utilities feature dataset and select Properties. Refer to XY Coordinate System tab in the Feature Dataset Properties window and your knowledge of projections from the last module to answer the following question:

Question 1

a. Is State Plane California Zone V in the northern or southern part of the state?
b. What projection is Zone V based on?
c. What are the units?
d. What is the datum?
  1. The next step is to create a feature class within the feature dataset you just created. Right-click on the Utilities feature dataset and choose New > Feature Class. Name the feature class pipes and give it an Alias (alternative name) Water pipes. This will be a vector layer with lines representing a pipe network, so chose Line Features for the type of features. Be sure that the Geometry Properties boxes are unchecked. Click Next.
  2. For the Configuration Keyword page, accept the default by clicking Next, which brings you to the attributes page.
  3. There are several attributes that we need defined for the pipes feature class.
    • In the Field Name column, first blank row (row 3), enter the value “Type”. In the Data Type column, click the cell below Geometry (row 3) and use the pop-up menu to select “Short Integer”. Left-click the cell so that window of Field Properties is visible, and change alias to “Line Type”.
    • Repeat the same steps to add a field called “maintDate” with a “Date” data type. In the Field Properties change the Alias to “Maintenance Date”.
    • Repeat the same steps to add a field called “maintType” with a “Text” data type. In the Field Properties change the Alias to “Maintenance Type”. Specify the Length field as “20”.
  4. Click Finish. Notice that the pipes feature class is added to your Table of Contents (TOC). There is nothing drawn in the map, though, because this feature class is empty – you will have to digitize the features.
  5. Right-click on pipes (under the Utilities feature dataset) and open Properties. Click the Subtypes tab. We will set up “subtypes” that link integer codes in a field to text descriptions via a lookup.
  • For Subtype Field, use the pop-up menu to select the “Type” field. Only “Type” is available because it is the only field that we created that has integer values. Set up the subtypes for the Type field in pipes using the table below. You do this by clicking in the Code and Description boxes and then type their values. Your subtype definitions should look like the table below.
Code Description
0 Unknown
1 Main line
2 House line
  • This process will control the values you are able to enter when you have digitized a new line segment.  When all values are entered, click OK to close the window and finish creating the feature class.
  1. Create another feature dataset in the City Geodatabase and name it LandUse.  Click Next.
  2. This dataset will use the same spatial reference as the Utilities feature dataset, so this time we’ll just import the coordinate system.  When you get to the Coordinate System screen, click the globe icon (across the top), select Import and browse to the Utilities feature dataset and click Add.
  3. Click Next, then click Next again and Finish to finish creating the LandUse feature dataset.
  4. Create a new feature class called parcels in the LandUse feature dataset. Right-click on the LandUse feature dataset you just created, go to New  Feature class, and name it parcels. This will be a vector layer with polygons representing parcels, so chose Polygon Features for the type of features. Be sure that the Geometry Properties boxes are unchecked. Click Next.
  5. Create attribute fields as follows:
    • Add a field called “APN” with a “Text” data type. In the Field Properties change the Alias to “Assessor parcel number”. Specify the Length field as “15”.
    • Add a field called “Cost” with a “Long Integer” data type. In the Field Properties change the Alias to “Land value (in dollars)”.
  • Add a field called “Acres” with a “Float” data type. In the Field Properties change the Alias “Size of parcel”.
  1. Click Finish.  Your City geodatabase should now look like this:
Geodatabase example

Importing attribute tables

To work well with the other files, the owners table (owners.txt) needs to be inside the geodatabase. 

  1. Right-click on owners.txt.
  2. Choose Export > To Geodatabase (single). This brings up the Table to Table window. The Output Location should be your City geodatabase.
  3. Type in owners as the Output Table name.
  4. Click OK.

Georeferencing

In order to digitize features in our new geodatabase, we are going to use a reference image and perform “heads-up” digitizing.  In order to digitize our new features correctly, the reference image has to be correctly positioned with regard to the surface of the earth so that our digitized features will likewise be correct.

Georeferencing the parcel.tif raster scanned map involves selecting control points on the image (source) that can be referenced to the real-world geographic or projected coordinate system (target). This can be done in two main ways, by matching points on the raster to points on another layer with a defined coordinate system, or by using GPS coordinates to define key positions on the raster.  Since we have no other layers, we’ll use the latter method.

  1. Start a new blank map.  Add the parcel.tif raster.  Open the Properties and gather the following information from the Source tab:

Question 2:

a.     What is the x and y cell size? (round to the nearest 4 decimal places)
b.     What is the “Spatial Reference” for the raster?
c.      What are the left, bottom coordinates (min x, min y)?
d.     What are the right, top coordinates (max x, max y)?
e.     What are the “Linear units”, if any?
f.      What are the linear units of the numbers displayed in the bottom-right when moving the mouse?
g.     Is ArcMap using these units correctly? Why or why not?
f. When you added parcel.tif an ArcMap warning box appeared.  Why, and what does it mean?
  1. Set the projection for your ArcMap session so that the georeferencing results are displayed correctly.  Import the coordinate system defined for our new feature classes to your data frame.
  2. Turn on the Georeferencing toolbar (Customize > Toolbars > Georeferencing).  Use the following image to reference the points on your image:
Georeferenced sample
Add control points tool
  1. Zoom into the first point (X1 in the image above). Click on the “Add control points” button on the georeferencing toolbar. Your mouse pointer will turn into crosshairs.
  1. Click as precisely as you can on the intersection of the two lines.  This sets the x-y point for the image.  To set the x-y coordinates on the ground, move the mouse slightly away from the point you just clicked.  You will see a connecting line.  With another reference layer, we could simply click on the same point on the other layer.  Since we are using GPS data, we will manually input coordinates.
  2. Right click and select Input X and Y.
Right click menu options

  1. The table below contains California State Plane coordinate data for each of the points.  Enter the X and Y values for X1 and click OK.  As you enter the data, the raster will move to fit the new coordinates.   Use the Full Extent button if it shifts off the screen.
Control point GPS X coordinate GPS Y coordinate
X1 1786095 624300
X2 1787416 624300
X3 1786481 623882
X4 1786738 624170
X5 1787371 623781
  1. Continue referencing and entering coordinates for all five control points.
  2. When you have entered all five control points, click on the View Link Table button in the Georeferencing toolbar.  Verify that these match the table above.
  3. On the Georeferencing toolbar, click on Georeferencing  Update Georeferencing to save the parcels.tif raster’s new, georeferenced coordinate system. Open the parcels.tif properties from the TOC to answer the following questions:

Question 3

a. What are the raster’s x and y cell sizes?
b. What is the Spatial Reference?
c. What are the linear units?
d. Which of these values are the same as the ones recorded Question 2? Which are different? Why are they the same or different?

Digitizing New Features

Now that you have a base map, you can start digitizing your new data.  Since you georeferenced the base map, your new data will be created in the correct coordinate system.  Be sure to complete the metadata for each of your new feature classes.

  1. Add the parcels feature class from your geodatabase and turn it on in the TOC.
  2. Turn on the Editor toolbar (Customize > Toolbars > Editor)
  3. Select Editor > Start Editing.  You should see a new Create Features pane appear on the right side of your screen.
  4. For easier digitizing, symbolize the parcels layer as hollow with a thick, bright border (red works well).
  5. To digitize a polygon for one parcel, first zoom in to the first parcel (parcel #20) in the upper left corner of the scanned raster. Click on the “Polygon” construction tool. Notice that in the Editor toolbar, a series of buttons are now available for “segment construction”.  Select Straight Segment.
  6. Digitize a polygon for parcel #20 by clicking in succession on the upper-left corner of the parcel, the upper-right corner, and the lower-right corner of the parcel. Notice that a triangular polygon has been created based on these 3 points.
  7. Finally, double-click on the lower-left corner of the parcel (below the diagonal of the triangle) to finish the polygon. Doing this will automatically snap the nodes and close the polygon. You should see a completed polygon highlighted with the familiar selection color and possibly a fill color if you chose not to make the polygon have a hollow fill. If you don’t see your polygon, make sure you turned it on in the TOC.
  8. SAVE NOW!  Click Editor > Save Edits
  9. If your polygon isn’t still selected, select it with the Edit Tool.  Click on the Attributes button on the Editor toolbar to enter attribute information for this parcel.  Notice the fields and values set by you when you created this feature class.
    • The Assessor Parcel Number (APN) for each polygon will take the form: 52-015-parcel#, where parcel# is the number in the circle inside each polygon.  In this case, the APN is 52-015-20.
    • Unpin the Attributes pane by clicking the pushpin at the top until it is laying on its side.  This will collapse the window until you need it again.
  10. Click on the Create Features button along the right edge of your screen.
  11. To avoid creating slivers when you digitize adjacent polygons, change the construction tool to Auto Complete Polygon.
  12. You will also want to enable snapping at this point.  Go to Editor > Snapping > Snapping Toolbar.
    • From the Snapping Toolbar, select the End, Vertex, and Edge boxes (Point is already selected by default). This will automatically “snap” the digitizing cursor to existing features.
    • Click the Snapping menu on this toolbar and make sure that there is a check next to “Use Snapping” – we want to use snapping properties.
  13. Digitize the remaining polygons using the Auto Complete construction tool.  You must start and end on the vertex, edge or end of an existing polygon.
    • Don’t forget to enter the APN attribute.  If you miss one, you can go back to it with the Edit Tool.
Arc Segment tool
  1. On parts of the map where there are curves in the polygon that you are digitizing, use the Arc Segment Tool instead of Straight Segment. You do this the first time by clicking the pop-up button in the floating Feature Construction Toolbar that appears when you digitize. Then select the Arc Segment tool.
    • Be careful!  The Arc Segment Tool and the End Point Arc Segment Tool look very similar.  Make sure you get the right one!
    • When you finish the arc, click back on Straight Segment to finish the polygon.
  2. When you are done with all polygons, open your attribute table and make sure each one has an APN value.

SAVE EARLY AND OFTEN!!

Calculating values

Your map is in State Plane, so the Shape_Area field that was computed when you created the polygons is in square feet. 

  1. Make sure no records are selected and use the Field Calculator to compute the area in acres (43,560 sq ft = 1 acre).
  2. Property in this subdivision is $300,000 per acre.  Use the Field Calculator to compute the Land Value field using this information.
  3. Save edits and stop editing  (Editor > Stop Editing).

Adding the Pipes network

  1. Turn off the parcels feature class.  Add the pipes feature class and turn it on.
  2. Change the symbology for pipes based on the Line Type field, using a distinctly different line for each type.
  3. Start editing.  In the Create Features window, click Organize Templates.
  4. Select New Template, which then opens a Create New Template Wizard window. Check Water pipe. Be sure parcels is unchecked (we already have a template for parcels from earlier in the lab). Click Next.
  5. Be sure all of the water pipe classes are checked, and click Finish.  Close the Organize Feature Templates window.  You should see all three pipe types in the Create Features window.
  6. Click on a pipe type and then use the Line construction tool to digitize your pipes.  Save often!  You are in charge of the network design! The only rules guiding your design are:
    • Main pipes can only run down the middle of streets. (You can consider the blank area at the top of the scanned map to be a street.)
    • Each parcel must have at least one house water line.
    • There should be no “unknown” pipe types when you are done digitizing.
  7. When you are done creating your pipe network, save and stop editing.

Making corrections

If at any point, you make a mistake or are not happy with the placement of a feature, use the Edit Tool to select it. 

  • You can delete selected features by hitting the Delete key on your keyboard
  • You can edit vertices of selected features by clicking the Edit Vertices button on the Editor toolbar, then individually moving or removing a vertex in your feature.

What to turn in

Create a word document with the answers to the questions. Make a map of your parcels and pipe network as follows, and add it to the document for submission to Canvas.

  1. Turn off the raster image
  2. Join the owners table to the parcels feature class and label parcels with owner names.
  3. Make the pipe types distinguishable from each other and from parcel boundaries.   Feel free to make the parcels a solid color now that you are done digitizing.
  4. Add a text box listing the projection information
  5. Add a table with parcel ID (APN), owner name, price and acres.
  6. Add relevant map elements.