chapter 4. gis and mapping 1. spatial data...

GIS and Mapping

Work Site Alliance – Community Based GIS 1 2000

Chapter 4. GIS and Mapping

GIS is not a computer system for making maps or graphics, but rather, a set of tools foridentifying “spatial” relationships between data features, or across data “themes”.Therefore, data structure is one of the first major decisions to be made in the GISprocess. Most GISs are either raster-based or vector-based, but many can employ bothformats. Raster can be transformed (changed) to vector, and vector to raster.

1. Spatial Data Organization• A GIS represents a set of real-world objects in a combination of various

themes or layers as map features, also known as digital map or digitalcartographic data (Diagram #1). In a single view, many themes can bedisplayed for the same geographic area (Diagram #2).

Diagram #1 Diagram #2

• Spatial data are stored along with some of their characteristics, calledattribute data, in the form of attribute tables. An attribute can be a non-spatial aspect of spatial data, such as name, classification, or color. Attributescome in many forms, including text descriptions, numbers indicatingquantities of some sort, and abbreviations or codes (for a name orclassification). (Davis, 1996)

• Selected layers from a GIS database are combined to create a map, knownas a composition.

In a composition, the choice of layers or themes depends on the needs of theuser and on the existing data in the GIS. Compositions may be viewed andstored in a computer, or printed out as maps.

2. Spatial Data CharacteristicsGIS spatial data allows you to display a variety of visible spatial characteristics(although sometimes seen with difficulty) on a map. Although the eye canestimate many of these properties, GIS provides highly accurate measures.• Area and perimeter are characteristics of features, and are easily calculated

in most GISs. The eye quickly appreciates the shape of features, butcomputers need special programming to make spatial form descriptions.

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• Spatial relationships between and among features can be important GISproject considerations and are not always apparent to the eye. Distance fromone feature to another is available through simple measurement. Also,proximity of features, such as connections or nearness, may be determinedthrough various measures to determine “neighborhood” characteristics.

• Time can be an important part of GIS data. The date of the data is meaningfulwhen determining trends or change. A given project may need periodical dataupdating, and obsolete data may cause confusion. In this case, it is importantto specify the studied period.

3. How ArcView Handles ThemesGeographic databases are the combinations of the themes for a given area. Thethemes in a geographic database can be used in concert to solve multipleproblems through multi-level analyses (ESRI, 1997). The design of a GISdatabase has to be flexible. For example, you want to be able to add newthemes, and delete old ones. In addition, you want to be able to separate themesto create new ones, and combine themes if they have common characteristics(Ibid).

Another important function of ArcView is that the link between raster or vectorfeatures in maps is dynamic. That is, there is a two-way relationship between themaps and the attribute tables that hold the information about them.

By changing information in a table, the respective mapped feature isautomatically changed as well. The true power of GIS is its ability to not onlydisplay, but to perform analyses on multiple themes at once.

4. Map DisplaysFigure 4.1 contains vector data displaying three separate layers of metropolitanDetroit. The bottom layer is the boundary census tract file for the City of Detroit.The second layer is the highway network, and the third layer is the Detroit PublicHigh School System. Each of these layers can be edited and manipulated, whichwill be discussed in more detail in the exercise.

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Figure 4.1. Detroit Public High Schools(Source: Michigan Metropolitan Information Center (MIMIC) Author: Michael Hart)

Figure 4.2 also uses vector data in a thematic map showing the percentage ofDetroit residents who voted yes for casino gambling. The geographic boundaryfor the population is from census tracts. Each of the census tracts has beencomputed by joining the population table and that of the polling results. Theresults have been color-coded in shades of green, as seen in the legend. Thedarkest green depicts areas where the polling results have the highestpercentages of “yes” votes while the lightest green depicts areas with the lowestpercentage of “yes” votes. This is one of the powerful spatial displays of a GIS,allowing an analyst to provide information to the concerned parties in a simplevisual.

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Figure 4.2. Casino Gambling Voting Map(Source: Center for Urban Studies, Wayne State Univerity)

5. From “Hard Copy” To Digital MapWe may wish to store, manage, and process in digital format some information ofinterest that we find reported on a “regular” map (i.e., drawn on paper). We thenhave to build a so-called GIS application. The paper map has to go through aconversion, from paper or analog form to number or digital form, which is thecomputer-readable format. This transfers the map data into the computer. Thisfile in turn must be converted to fit into either a raster or vector GIS. Thisconversion process is usually called geocoding. There are two major methods toconvert hard copy maps into digital format. One is called digitizing, and another iscalled scanning.

5.1 Geocoding MethodsGeocoding by tracing over a map with a cursor is called digitizing. The devicesused for digitizing are a digitizing board or a tablet and a cursor. The digitizingboard is comprised of tiny wires that run horizontally and vertically inside, forminga grid. The digitizing cursor has an optical viewer with crosshairs that allows the

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user to visually locate a point on the map. The most common cursor has at least16 buttons and contains a keyboard that can be adjusted to the user’spreference. Digitizing converts spatial features on a map into a digital format. Todigitize, a map is mounted on a digitizing board and a digitizer cursor is used totrace each map feature. When a point is identified on the map, the user pressesa button on the cursor and the tiny wires receive an electrical impulse. Then thecomputer records the current X, Y coordinate location of that position in digitizerunits (usually centimeters or inches). This becomes the X, Y coordinate of thepoint feature, or one of the points comprising a line or polygon.• The second technology used to perform geocoding is called scanning. The

drum scanner is most commonly used for map entry. This type of scannerreceives an entire sheet map, usually clamped to a rotating drum, and scansthe map. Now, however, flatbed scanners are becoming more widely used inthis geocoding method. The output of map scanning is an image data file,also called a raster data file. In most cases, these raster data should beconverted into vector data formation. This process is called raster-to-vectortransformation.

5.2 Digitizing ProceduresTo convert hard copy maps into digital form by the digitizing method, theoperation procedure is software dependent. Regardless of which software youuse, however, there are three steps you should follow.

Step 1: Use a good base map

Digitizing a poor quality map directly affects the accuracy of your digital data,resulting in a misrepresentation of the geographical data. You should use paperdocuments in good condition, clean, easy to read, and not torn or folded. It isthen recommended to copy the paper map onto a stable material if necessary.

Step 2: Prepare base map

Any features that are selected for digitizing should be marked in advance. Thebase map is prepared by choosing the control points (usually at least 4 controlpoints for each map) at known locations on the hard copy, such that theircoordinate pairs into a real world system, like UTM or State Plane CoordinateSystem, can be documented from other sources. These control points will thenbe used to transform the entire set of coordinate pairs from the digitizer units intothe real-world coordinate system.

Step 3: Digitize your map

Before digitizing, the map must be taped onto the digitizing table. First, thecontrol points are digitized and then map features are traced out using thecursor. You should be careful to capture features accurately with a suitable levelof detail. Usually points are entered one at a time. Lines are entered as strings ofpoints, and must be terminated with an “end-of-chain” signal to determine whichpoint forms the node at the end of the line. Areas are digitized as groups of linesthat close to form a polygon, with a point (called a label point) within the polygonto identify it. An important note; for the line feature, start/end points, which are

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called nodes, are different than the points between nodes, which usually arecalled vertex points.

5.3 Data editingIn most cases, there are always errors in the output data set that was created bydigitizing. The process of correcting the digitizing errors is called data editing.There are two kinds of digitizing errors: node errors and label errors.

• Node errors include two types of potential errors: pseudo node and danglingnode.

Pseudo nodes occur where a single line connects itself (an island) or where onlytwo arcs intersect.

Figure 4.3. Examples of pseudo node errors. (Source: Arc Online Help)

Dangling nodes connect to only one arc, or are the unconnected nodes ofdangling arcs. Dangling arcs and nodes are created when digitized arcs stopshort of, or extend past, an intended intersection point.

Figure 4.4 Examples of dangling node errors. (Source: Arc Online Help)

Pseudo nodes do not necessarily indicate an error or a problem. Acceptablepseudo nodes may represent an island or the point where attribute valueschange along a linear feature (e.g. road type changes from pavement to gravel).In some cases, dangling node may also be acceptable; (e.g., in a streetcenterline map, dead-end roads are represented by dangling nodes).

• Label errors occur either when a label is missing or when more than onelabel point is within the area. There should be a unique label point within eacharea (polygon) to identify it. The methods of data editing depend on thesoftware you use.

5.4 Data transformingThe features from the digitized map are not often in the same coordinate systemas the data in other themes that we want to display on the same View document.This is corrected by first setting the control points into some real-world coordinatesystem, such as UTM or State Plane. Then the entire digitized data set will beautomatically referenced in that same coordinate system. Otherwise you musttransform the digitizer units, which usually is centimeters or inches, to a real-world coordinate system. This process is called data transforming.

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6. Using Tabular DataIn ArcView, any tabular data containing XY coordinates can be displayed on aview as point features. In addition, tables containing references to geographicfeatures such as street addresses can be given XY coordinates in ArcView bygeocoding. This means that in most cases you don't need to manually digitizegeographic data that is tied to point locations. Instead, you can display yourexisting tabular files directly on a view.

Spatial data sources such as ARC/INFO coverages or ArcView shapefiles haveattribute tables containing descriptive information about their features. You canwork with these tables in various ways. For example, existing tabular data can bejoined to the attribute tables of spatial data sources. This lets you represent thespatial features according to the values they have in the fields of your own tables.It also allows you to select features according to these attributes.

NOTE: Spatial data structures are relevant to this discussion because givenattribute information is “hooked onto” the spatial data according to the datastructure. Remember that raster cells have specific size and location, and thatvectors have length between nodes, and a direction that gives us the ability touse the right and left side of the line segments.

6.1 Working With Event Tables in ArcViewEvent tables enable you to map data that contains geographic locations butwhich are not in a spatial data format. For example, you might have a file ofcustomers and want to make a map showing how they are distributed throughoutthe region. You may have a table of accident information and want to display theaccidents with reference to their milepost locations along a route. Or you mighthave a table with information about bird sightings, with the latitude-longitudelocation of each sighting as fields in the table. In ArcView these are all examplesof events (ArcView Online help).

6.2 Types of Events That ArcView Supports

XY event tables

These tables contain the exact position of features as X, Y coordinates, such asthe location of fire hydrants in a city or the points where soil samples have beentaken. X, Y coordinates can be obtained by reading a map, measuring positionson a view, field surveying, using a GPS (Global Positioning System) receiver,geocoding the table that contains address information, etc. The X, Y coordinatescan be in any coordinate system and units, such as latitude and longitude ormeters.

Point event tables

These tables contain the locations of points specified as positions on a line. Forexample, a point event table might record the location of bus stops along a busroute defined by their distance from the start of the route, rather than their XYcoordinate location. Example: "At 12.5 miles along Route 10".

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Linear event tables

These tables define events as segments along a longer line. For example, alinear event table might record the pavement quality along different sections of ahighway, where the sections are defined by distance along the highway. This isalso a useful method for recording continuous data, such as the location of speedzone changes along a highway, with the location defined by distance along thehighway.

Address event tables

Address event tables contain location identifiers (street addresses, or any other):customers, suppliers, competitors, branch offices, facilities, crimes, etc.

NOTE: In order to add a point or a line to a view, the view must already contain atheme with the appropriate reference system (such as a street network, forexample). ArcView uses this reference theme as the base map on which to mapthe events. In order to add address events to a view, the view must alreadycontain an appropriate theme on which a new address can find a match. Then,ArcView can use this reference theme as the base map for geocoding the table.

7. Join & Link

7.1 Joining TablesThe ability to join tables based on a common item is one of the most importantfunctions in database management. Simply stated it allows for non-redundantdata storage and simplifies database management (Hutchinson and Daniel,1997).

For example, a data file that contains the inventory of a school district caninclude an address for every student in the district. A look-up table can then beprepared associating each student record with that student’s current grade level.Editing the much smaller look-up table ensures that the grade level changes willbe subsequently applied to each student. The two major benefits that derive fromthe use of joined tables are (1) when changes occur, only one file has to beupdated; and (2) because the link between joined tables is dynamic, subsequentviews of the joined table will reflect changes after a file has been changed (Ibid).

ArcView’s join function is particularly powerful, allowing tables from differentsources to be joined, stored in the computer, and visualized onto the screen as avirtual table. The source file that is to be joined to the spatial theme attributetable can be any of the table formats discussed earlier. The mechanics of how toperform a join are straightforward, and are covered in Exercise 4C at the end ofthis chapter. A related concept, namely the relationship between the source tableand the destination table, remains to be discussed.

In ArcView, the destination table is the table to which the fields from the sourcetable will be appended (Ibid). Typically the destination table is the attribute tablefor the spatial theme. The results of a join are accurate only if there is a one-to-one correspondence between records being matched from the source to the

GIS and Mapping


destination table; that is, only one unique record in the source exists for eachrecord in the destination. The relationship may be one-to-one, as in the linkbetween a land parcel theme and the associated parcel data. The relationshipmay also exist as many-to-one, as in the link between a student theme, and thelook-up table explaining information about each student (Ibid).

7.2 Join versus LinkIf there are many records in the source table that link to the destination table,only the first record from the source table will be joined to the destination table. Inthis situation, you should be linking rather than joining the tables (Ibid). The linkfunction is used when a many-to-one relationship exists between a source tableand a destination table. Linking the two tables ensures that all records from thesource table are associated with, but not actually joined to, the destination table(Ibid). The many source table records will subsequently be available when youquery the spatial theme. This will become apparent when you work throughExercise 4C. In that exercise, we see how tables are connected through the linkfunctionality in ArcView, without actually appending the fields of the source tableto the destination table.

Exercises

4A. Editing tablesIn ArcView, when you edit data in a table (change field values, add or deleterecords and fields) the source data for the table is modified. This also means thatif you add the modified table to a different project, your changes will be reflected.But when changes are made to the appearance of the table (renaming, hidingfields, or changing the fields display width) these changes apply only to theproject in which they are made.

Step 1: Change table display

Now you will make changes to the appearance of the table Children.dbf.

Open the project. Your Drive:\chapter 4\ex04a.apr.

Make the Children.shp theme active.

Open the Attributes of Children table.

From the Table menu, choose Properties to open the Table Properties dialogbox.

The name of the table appears at the top of the dialog box. At the bottom of thedialog box are three columns. The first tells you whether a particular field isdisplayed or hidden; the second is the field name; and the third lets you “rename”the field by assigning an alias to it.

In the Visible column, click on the check mark by the Av_status, Av_score, andAv_side fields to hide them in the table display.

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Then click in the Alias column and locate the Id field and type User-id as analias.

Click OK to apply your changes.

The Id field is now called User-id, and the Av_status, Av_score, and Av_sidefields are hidden.

Step 2: Edit values and add a record

Suppose you find that the reported birthday for the child named Adolph KevinAnthony is wrong (see record 10, after you sort the Child_n field in ascendingorder). It should be 02/15/95 instead of 02/14/95. Your are called to correct thiserror.

From the Table menu, select Start Editing.

To sort the Child_n field, highlight the field name.

Then click on the Sort Ascending button:

NOTE: There is also a Sort Descending button as well.

Click on the Edit tool.

Click in the cell of record no.10 and field Birth, the value is highlighted. Type inthe right value: 9950215, then press <Enter> key.

Now you will add a record to the table.

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From Edit menu, choose Add Record.

Scroll to the bottom of the table. A new record is appended to the table. It haszeros in the numeric fields and is blank in the character fields. You can use thesame method mentioned above to set values to the new record.

Step 3: Add a field and calculate values

You may have noticed that the values in the Birth field have an extra digit (9) thatis not necessary for our table. Now you will add a new field called Birth_day tothe table and set the correct values to it.

From the Edit menu, choose Add Field to display the Field Definition dialog box,set the Name as Birth_day, Type as Number, 16 for width, and 0 for DecimalPlaces.

Click OK to add the new field to the table.The new field is active in the table.

Click the Field Calculator button.

The Field Calculator displays:

In the Fields scrolling list, double-click on [Birth] to add it to the text box. In theRequests scrolling list, double-click on “-“ (minus), then type 9000000, to makethe expression [Birth] – 9000000. Then click OK.

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Now, each cell in the new field contains a date value, which is formatted asyymmdd.From the Table menu, select Stop Editing.Click Yes to save your edits.

Step 4: Close and save the Project

Make the Project window active.From the File menu, choose Save As… to save the project in your personaldirectory. Then choose Close Project to close the project.Click No to discard changes.

4B. Query tables

Step 1: Start ArcView and open a project

If necessary, start ArcView.

From the File menu, choose Open Project.Move to YourDrive:\chapter 4\ and open ex04b.apr

When the project opens you see a view that contains a point theme calledChildren.shp and a polygon theme Detroit_zip.shp. Each point stands for achild and his/her family house location.

Make the Children.shp theme active and open its attribute table.

Step 2: Query on a table and display the selected set

The first task you should do is to find out the kids who were born in 1995.

With the table active, click the Query Builder button.

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The Query Builder dialog box displays:

Now you will use Query Builder to select records in the theme table and thecorresponding features in the view. In the Fields scrolling list, double-click on[Birth] to add it to the query text box.

From the logical operators, click the “>=” button. Then type “9951007”Finallyyou should build an expression like as “([Birth_] >= 9950919) and ([Birth_] <=9951007)”.

Click the New Set to select all children who were born in 1995.

Make the table active by clicking on its title bar, then click the Promote button to move the selected records to the top of the table.

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On the left side of the tool bar, you see the total number of children satisfying thecondition of the search (in this case, 2 records out of 150). Also, the selectedrecords in the attribute table and the corresponding points in the view becomehighlighted.

Step 3: Summarize a table

By summarizing a table, you count the occurrences of each unique value in aspecified field. Optionally, you can get statistical information on another field inthe table. The results of the summary are stored in a new table that ArcViewcreates.

Suppose your are asked to find out how many children are in each zip area.Make the Attribute of Children table active, and click the Select None buttonto clear any records that have been previously selected.

In the table, click on the Zip field to make it active.

Click the Summarize button to open the Summary Table Definition dialogbox.

In the Save As… input box, change the pathname to that of your personaldirectory, such as Your Drive:\yourname\sum1.dbf. Set Field: as Shape, andSummarize by: as Merge.

Click OK.

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A new table, called sum1.dbf, displays. It shows you the number (Count) ofchildren that belong to each zip area.

In our example, it shows that one record is not assigned a zipcode. If needed, wecan identify this record (by sorting the zipcode field in ascending order), andperform the desired editing.

Step 4: Save and Close the project

Make the Project window active.From the File menu, choose Save As… to save the project in your personaldirectory. Then choose Close Project to close the project.Click No to discard changes.

4C. Join and link tables

Step 1: Start ArcView and open a project

(same procedures as the corresponding section in Exercise 4b, but openex04c.apr).

Step 2: Add a table to the project

From the Project menu, select Add Table.

In the Add Table dialog box, move to Your Drive:\chapter 4\data

Click on children_plus.dbf to highlight it. If you do not see children_plus.dbf,make sure dBASE is selected in List Files of Type.

Click OK to add the table to the project.

Step 3: Join tables

In ArcView, the join tables operation temporarily appends the fields of one table(the source table) to another table (the destination table). To join two tables, thetables must share a field of common information. ArcView uses the values in thisfield to match the records in the two tables. It is important that the relation ofrecords between the destination and source tables be one-to-one or many-to-one.

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Now, you will join children_plus.dbf to the Attributes of children table.

Make the children_plus.dbf active, check all its fields and click on the fieldChild_n to make it active. Table children_plus.dbf is the source table in thisexercise.

Open the Attributes of children table and make it active.Make the field Child_n active.

Click the Join button .

Now, do you find any changes of the Attributes of children table?

From Table menu, choose Remove All Joins, and notice the changes to thetable.

Step 4: Link tables

When the relation between your destination and source tables is one-to-many orone-to-one, you should link rather than join the tables. Like a join, a link requiresthat the two tables share a common field. When tables are linked, selecting arecord in the destination table selects all records in the source table that have thesame value for the common field.

Now let’s link the Attributes of children table to the children_plus.dbf table.

Make the Attributes of children table active and make the field Child_n active.Make the children_plus.dbf active and make the field Child_n active.From Table menu, select Link.

(The two tables are now associated, but no fields are appended to thedestination table and the source table remains open).

Make sure the Select Record tool is available, and click on it.

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Select any record in the children_plus.dbf table.You will find that the corresponding record(s) are selected and highlighted in theAttributes of children table.

With the children_plus.dbf table active, from Table menu, select Remove AllLinks.

Using Select Record tool to select any record in the children_plus.dbf table.No record(s) in the Attributes of children will then be selected.

Step 5: Save and Close the project

Make the Project window active.From the File menu, choose Save As… to save the project in your personaldirectory. Choose Close Project to close the project.Click No to discard changes.

4D. On-screen digitizingIn this exercise, you will understand the basic concepts of map digitizing throughscreen digitizing - a virtual digitizing environment and process.

Step 1: Start ArcView and open the exercise project

If necessary, start ArcView.

From the File menu choose Open Project.

Move to the directory Your Drive:\chapter 4\ and double click the projectex04d.apr. When the project opens, you see a view containing theDetroit_zip.shp theme. This time, we will use it as the source data to be digitizedinto the computer.

Step 2: Create a new shapefile as a theme

From the View menu, select New Theme to create a new theme. In the New Theme dialog box, select Polygon as feature type, then click OK.

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In the New Theme file browser, move to your personal directory and save thenew shape file as Zones.shp

The new shapefile theme Zones.shp is added to the table of contents in theView document, and is ready for editing (note the dashed line around its checkbox).

Step 3: Set the snapping environment

With the use of the snapping environment in ArcView, you can improve how newfeatures meet and align as you create them. (For more details on the Snapfunction, please consult the Online Help for ArcView).

Click on the Zoom In tool and then drag a box in the area of the view to zoom ina small area (the smaller the area you zoom in, the smaller the snap tolerancecan be).

Place the cursor in the obtained view and hold down the right mouse button todisplay the popup menu. Move the mouse down to highlight Enable GeneralSnapping, then release the right mouse button.

Click on the General Snap tool

Place the cursor in the view. Click and drag out the circle attached to the pointer(as the size of the circle varies, you can read the radius values in the bottom leftside of the ArcView screen). You stop dragging the circle once you read thedesired radius value (e.g., 0.01 miles) that then becomes the general snaptolerance.

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Step 4: Add features and attributes to the new theme

If necessary, use Zoom In tool and Pan tool to zoom the Detroit_zip.shp themeto the desired display scale. You will add several polygons to the Zones.shptheme by subdividing the map area into “East”, “Center”, and “West”. For eachpolygon you draw, you will add the zoning code to the theme attribute table.

Open the Zones.shp theme table.

Note that there is no record in the listed fields. You will add a new field calledZone to this table. From the Edit menu, select Add Field to display the Fielddefinition dialog box. Fill it as displayed in the following figure. Then click OK.

Make the View window active.

From the Draw tool drop-down list, choose the Polygon tool.

Now move the cursor (Notice that it changes into cross hairs) into the view. Drawa polygon: each click creates a vertex point, double-click to complete thepolygon.

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Make Zones.shp the only active theme. Double-click on it and proceed to thesetting as mentioned below.

By default, the Zones.shp theme has a solid fill pattern. Polygons you add to thistheme cover up the source map (Detroit_zip.shp) beneath these new polygons.You should then change its fill pattern from solid to transparent (first white box inthe Fill Palette), and a bold outline (select 2 in the Outline dropdown list, in FillPalette).

Make the theme table active. A record has been added for the polygon you justdrew. Click the Edit tool,

then click on the record under the Code field and type East as the zone name.

Next, you will draw a new polygon adjacent to the previous one (for Centerzone).

Make the view active.

From the Draw tool drop-down list, click on the AutoComplete tool.

This tool lets you draw a line to append a new polygon to an existing, adjacentone.

Draw a line starting at (or inside) the boundary of the existing polygon (codeEast). Click to enter each vertex; double-click to end the line at (or inside) theboundary of the existing polygon.

NOTE: The two polygons will share a common boundary; you do not need todraw this boundary.

Now make the theme table active and enter Center as the second zone name.

Repeat the above steps for the last zone that we name West.

Step 5: Edit features using Split and Union

You will use the Split tool to split the zone coded “West” into two polygons, thenjoin one of them to the polygon coded “Center”.Before you edit the polygons, you will specify how ArcView will update theirattributes in the theme's editing properties.

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Make the view active, with the Zones.shp theme active.

From the Theme menu, select Properties to display the Theme Propertiesdialog box. Click on the Editing icon.

In the Attribute Updating portion of the dialog box, click on the Field drop-downlist and choose Zone, then click OK.

Note: The Zones.shp theme table should still be open so you can see howattribute values are updated when you perform the split.

With the view active, click on the Draw tool drop-down list and select thePolygon Split tool.

Click once to start the line. (The line should start and stop outside -overshoot- theboundary of the polygon you want to split.)

Double-click to end the line. The polygon is now split into two polygons.Next use the Union Features tool to merge one of these new polygons with thepolygon coded Center.

Click on the Pointer tool, then click inside the polygon center to select it.

Hold down the <Shift> key and click inside the polygon adjacent to polygonCenter. Now two polygons are selected.

With the view active, click the Edit menu and select Union Features.

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The two polygons are now merged into a single feature with a signal record in thetheme table.

Step 6: Stop editing and save your edits

From the Theme menu, select Stop Editing.

When you are prompted to save your edits to Zones.shp, click Yes.

Step 7: Save and close the project

Activate the Project window and, from the File menu, choose Save Project As.Move to your personal directory and save the project as a new project name.From the File menu, choose Close Project.

4E. Heads-Up Digitizing and Creating DataIn this exercise, you will learn how to create your own data by digitizing off atopographic map that has already been brought into ArcView and given aprojection and coordinate system. Digitizing, as described earlier in the chapter,is a way to get data from a paper map into the GIS. Sometimes there will not besufficient data for a project and some will need to be created. You are going tolearn how to create your own data using heads-up digitizing. Heads-updigitizing is similar to tracing off a base map. It does not involve using a digitizertablet or special mouse; just the ArcView program and a base map off of whichyou will digitize.

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Step 1. Start ArcView and open the exercise.

From the File menu choose Open Project.

Move to the directory: Your Drive:\Chapter 4\ and double-click the project fileex04e.apr.

When the project opens, you see an empty view. Click on the Add Theme buttonand navigate to Your Drive:\Chapter 4\data

Under the Data Source Type select Image Data Source. Highlight Ypsi_east.tifand click OK.

The image will be added to the View. Click on the Ypsi_east.tif theme to make itactive.

Notice the different colors associated with the theme. The major highways androads in the theme are red, lakes and ponds are blue, urban areas are reddish.

Zoom in to the area of the map that is urban. Notice the schools, hospitals, andpost office are points on the map as seen below.

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Step 2. Create a New Line Theme

To create a new theme, you first must decide whether it is to be a point, line, orpolygon feature. Let’s digitize the roads first, so that would be a line feature.

Under the View Menu, choose New Theme. A box will display asking which typeof theme you want. Select Line.

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Give the new theme a name and put it in your personal directory:Your Drive:\yourname\roads.shp.

The new roads.shp theme has been added to the view. The box around thecheckmark means you re in edit mode and can begin digitizing:

Click on the Draw Line tool. The cursor will turn to crosshairs.

Begin digitizing along highways and those roads depicted with a heavy red line atone edge of the map by moving the cursor along the feature, clicking periodically.When you are digitizing, it is important to be accurate. Zooming in and zoomingout is necessary to see the details as well as the “bigger picture.” Click one timeand then move the mouse and click again. Continue clicking until the line iscomplete as shown below. Then, double-click to end the line.

NOTE: If you zoom in few times it will make the digitizing easier. But this meansthat actually following the road is hard because it is segmented. When you havebegun digitizing, right-click on the mouse to access the following menu:

Select Pan, this will move the base map over so you won’t need to stop and startas much, giving you a continuous line. Also in this menu is the Delete Last Point

GIS and Mapping


tool. If you decide that the last point you digitized is not correct, right-click andselect Delete Last Point. You can use this feature a number of times in a row todelete more than one point. There are also options for Zoom In and Zoom Out.You can right-click in the middle of digitizing a line to make sure you are in thecorrect location. By using this right-click menu, the digitizing process is notinterrupted, leaving the data fragment free.

Continue digitizing more of the highways on the rest of the base map.Periodically, save your edits by going to the Theme menu and selecting SaveEdits.

If you digitize all of the major highways, the roads.shp theme will look similar tothe one below:

Under the Theme menu select Stop Editing. Select Yes when asked to saveyour edits. The roads.shp theme is complete.

Now we need to add values to the attribute table associated with roads.shp.

Click on the Open Theme Table button. Under the Table menu select StartEditing. This enables us to make changes to the table. Now we need to add afield to identify the different roads that were digitized.

Under the Edit menu choose Add Field…

Make the following changes in the Field Definition Box:

Name: NameType: StringNumber: 16 (default)

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The new field Name is added to the table. Return to the View and select one ofthe lines.

Return to the Attributes of roads.shp by clicking Open Theme Table.

There will be one record highlighted. Click on the Edit tool and enter the name ofthe road into the Name field.

Select another road in the View and add the name to the new field. Continue thisprocess until all records contain names.

Other fields can be added to the Attributes of roads.shp table. A field nameType can identify the different types of roads (i.e. interstate, primary, secondary,etc.) Add more fields on your own for practice.

Select Stop Editing under the Table menu.

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Step 3. Create a New Point Theme

The area south of Ford Lake in Ypsilanti Township is steadily growing. Many newhomes are being built in large subdivisions. Let’s use these new homes as ourpoint theme.

Under the View menu, select New Theme but this time we will choose point forthe Feature type, to digitize new buildings. Click OK.

Give the new theme a name and put it in your personal directoryYour Drive:\yourname\buildings.shp.

Double-click on the buildings.shp to open the Legend Editor to change thelegend from a dot to a square. Choose a square dot instead of a circle by doubleclicking on the symbol in the Legend Editor. Choose a different color. Be sure toclick Apply before closing the Editor.

Find an area south of Ford Lake that has purple points. This indicates newdevelopment since the last time the topographic map was updated.

GIS and Mapping


Zoom in closely to the purple subdivision:

Choose the Draw Point tool and begin digitizing the different houses in the newsubdivision.

Continue to digitize all of the houses, remembering to save edits periodicallyunder the Theme menu. When you are finished digitizing, choose Stop Editingunder the Theme menu. The buildings.shp theme is complete.

We can add the addresses of the houses we digitized to the Attributes ofbuildings.shp. Open the table and start editing.

Add a new field under the Edit menu.


Name: Street_AddressType: StringNumber: 25

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In the View, choose one of the buildings that you digitized. Return to theAttributes of buildings.shp and enter the Street_Address field. For thepurposes of this exercise, arbitrary addresses are entered.

Continue to select different buildings and enter addresses.

Other fields can be added to the Attributes of buildings.shp table. A field nameType can identify the different types of buildings (i.e. commercial, residential,industrial, etc.) Add more fields on your own for practice.

GIS and Mapping


Select Stop Editing under the Table menu when you have finished addingaddresses to the table. Be sure to save your edits.

Step 4: Create a New Polygon Theme

For this step, we will be digitizing Ford and Belleville Lakes.

Under the View menu, select New Theme and choose polygon for the Featuretype, to digitize the lakes. Click OK.

Give the new theme a name and put it in your personal directoryYour Drive:\yourname\lakes.shp.

Zoom in to the area surrounding Ford Lake. Select the Draw Polygon tool.

Begin at the northern part of the lake where the Huron River enters. Follow thedarker blue boundary around the lake until you reach the starting point again.

Start a new polygon for Belleville Lake. Once you are done digitizing, save youredits and select Stop Editing under the Theme menu.

Open the Attributes of lakes.shp so we can add a new field to name the lakes.

Select Start Editing under the Table menu.

Add a new field under the Edit menu.

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Name: NameType: StringNumber: 16 (default)

We only digitized two lakes in this theme. In the View theme, select one of thepolygons. Return the Attributes of lakes.shp and enter the name of the lake inthe new field. Repeat this process with the other lake.

Other fields can be added to this table, including lake depth, area, perimeter, etc.

Select Stop Editing under the Table menu when you have finished adding thenames of the lakes to the table. Be sure to save your edits.

NOTE: To add more points, lines, or polygons to the themes, select StartEditing from under the Theme menu.

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Now that we have digitized a point theme (buildings.shp), and line theme(roads.shp), and a polygon theme (lakes.shp), take a look at them together:

Step 5: Save and close the project

Activate the Project window and, from the File menu, choose Save Project As.Move to your personal directory and give the project a new name.From the File menu, choose Close Project.

GIS and Mapping


BibliographyDavis, David, 1996. GIS for Everyone. ESRI Press, 156pp.

Environmental Systems Research Institute (ESRI), 1997. ArcView Online Help.

Environmental Systems Research Institute (ESRI), 1997. ARC/INFO Online Help.

Michigan Metropolitan Information Center (MIMIC), Michael Hart, author of DetroitPublic Schools Map.

Hutchinson, Scott and L. Daniel, 1996. Inside ArcView GIS. Third Edition. On WordPress, 544pp.

Wayne State University Center for Urban Studies, 1999.

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