20

Buffer Zones

Buffering is the process of creating a new area feature or features (buffer zones) that encompass an area around a point, line, or other area feature within a specific distance or distances. Buffering may be the most common spatial operation performed in a GIS or desktop mapping system. GeoMedia has a number of innovative buffering techniques and features, which are covered in this section.

The product of a buffering operation is a buffer zone, which is one (or more) new area features. Because new features are being created, you must have access to a writable warehouse (i.e., Access). You do not have to write area buffers into the warehouse serving as the source of the features you are buffering. For example, you might have only read access to a warehouse of streets, but you could create a new warehouse of your own and place the results of buffering of the streets into it.

When you create buffer zone features, you may want to place them into a new feature class or simply add them to other existing features in a previously created feature class. The process of buffer zone feature class definition is automated by GeoMedia. It is one of the few cases in which you do not have to define a new feature class first.

There is a slightly different process for creating buffer zones around points, lines, or areas. The first thing you need to do is choose the features around which the buffer zones are to be created. All features in the selected group will be buffered. The group can simply be a feature class (e.g., all oil wells in the project), or you can perform a query process to create a query set and buffer the feature instances in the set (e.g., "All wells producing 1,000 barrels that are in Smith County"). The selection process will have to be performed before you start the buffer zone creation. Query procedures are described in Part V.

You must have a map window open to be able to create a buffer zone. If there is no map window open, the Insert button on the main menu does not have a Buffer Zone option. When the buffer zone is created, it is automatically displayed in the current map window.

Once you have selected the feature class or query set to use, GeoMedia automatically determines what type of features are represented (points, lines, or areas), and the options available in the buffer zone dialog change based on the feature geometry.

Creating a Buffer Zone
Around Point Features

To create a buffer zone around a point feature, select Insert > Buffer Zone from the main menu. The "Point buffering" dialog, shown in the illustration at left, will be displayed.

You need to indicate where you want to output the resulting buffer zones. The warehouse is selected from the Warehouse dialog, and all available warehouses can be viewed with the scroll box on the right of the dialog.

2 NOTE: The warehouse must be writable. The feature class name to be assigned to the resulting buffer zones is entered into the Feature Class box. This can be a new name if the results are to be placed into a separate feature class, or it can be the name of any existing area feature class. Each buffer zone produced is a unique feature class instance and has its own ID number assigned automatically. 2 NOTE: Feature class names cannot have any blanks or special characters, except for underscores. The Style button takes you to the Legend Style dialog, which you use to create a special style for the buffer zones to be displayed. The buffer zones are automatically displayed on the current map window when the process is completed. Using the style button now may save you having to modify the legend at that time.

When the feature class you are buffering from consists of points, you have a wide number of options. You can create a single zone at one distance, or multiple zones at a set of distances (either circles or rings). You can also have the resulting areas merge or not merge if they happen to overlap.

Single-distance Merged and Unmerged Buffer Zones

The basic type of buffer zone is a single "circle" around a single point. To create a single circle around point features, select Single from the drop-down list in the Point dialog area of the Buffer type section of the Buffer Zone dialog. If there is more than one point in the feature class or query set you are buffering from, there is the possibility that the buffer zone associated with one starting point may overlap the zone from another starting point.

You have two options for correcting this situation: to have the resulting zones be independent feature instances or to merge the overlapping ones into a single instance. The radio buttons at the bottom of the Buffer Zone dialog allow these options to be chosen. The following illustration shows the result of a single buffer zone operation where the results are unmerged.
 

The four starting points in the illustration at left lead to four buffer zones instances. Each of these instances has its own entry in the warehouse. Although the point is shown on the map window for visual reference, the point is not part of the resulting buffer zone.
 
 

The illustration at left shows the result of an identical buffer operation on the same four points. However, in this case the overlapping zones (if present) were to be merged. This illustration uses the same starting points and buffer zone distance used in the previous illustration. The only difference is that the merge option was selected. As a result, there are only two feature instances produced: the lumpy one that surrounds the three points and the single one around the single point. In this case there are only two new records in the warehouse.

Multiple-distance Ring Zones Around Point Features

Single buffer zones are easy to understand, but it is commonly the case that multiple zones of distance are very useful, an example of which would be "How many student residences are located 0 to 1 km from the proposed school, at 1 to 2 km, at 2 to 3 km, and at 3 to 5 km?" In this example, there would be four buffer zones. Each would be an individual ring, like a donut, around the point that represented the school. Together there would be four of these concentric rings.

The first of illustration at left shows an example of the results of the creation of buffer zones at 2 and 3 kilometers from the starting points. Here, there were eight area feature instances created from the initial four points. Four of the buffer zones were the circles out from the starting point, and four were the donut from 2 to 3 km. These multiple rings are created using the Multiple Rings option available in the Point drop-down list. After you have selected the Multiple Rings option, you would then enter 2 and 3 km in the Constant Distance dialog area located at the upper right of the Buffer Zone dialog (shown in the second illustration at left). Be sure that the proper distance units are entered.

2 NOTE: Ring (and stacked) zones cannot be merged. Therefore, the Merge radio button is not available when you have selected either of these options. Multiple Stacked Zones Around a Point

An alternative form of zones around a point is represented by stacked circles. In the stacked circles, the larger circles encompass the area of the smaller ones. In the previous example of 0 to 2 and 2 to 3 km zones, the first zone was a complete circle (with its center at 0 and a radius of 2 km), and the second zone was a donut or wheel (with an inner radius of 2 km and an outer radius of 3 km). With the stacked circle option, the first zone is the same as with the concentric ring option; that is, a circle with the center at the starting point and a radius of the first distance (in the example, 0 to 2 km).

The second, larger zone in the stacked circle option also has its center at the point of origin, and extends outward to 3 km. In other words, the area covered by the inner circle is also represented in the larger area, which is not a donut but a complete circle. If there had been additional larger zones, they would also have encompassed all of the smaller, inner ones. Chapter 17 noted that GeoMedia does not require planar enforcement of area features. Therefore, the same location can be present in two (or more) area features in the same feature class.

Stacked buffer zones are one example of the benefits of this approach. In the discussion of multiple ring zones, the example question was "How many children live within 0 to 1 km of the school, 1 to 2 km, and so on." Stacked circles would be used to answer a different but similar question: "How many children live within 1 km of the school, within 2 km of the school, within 3 km of the school, and so on?" In each case, the students that live within 1 km would also be counted with the students that lived within 2 km, and so on.

The illustration at left shows the output from a buffering using the stacked circles. Note that the larger (outer) circle is diagonally hatched to the left, and that the inner one is hatched to the right, so that the inner circle appears to be cross-hatched because it is part of both the inner and outer areas. There are eight feature instances that were created in the example, with eight records in the warehouse.

2 NOTE: Stacked (and ring) zones cannot be merged. Therefore, the Merge radio button is not available when you have selected either of these options. Structure of a Buffer Zone from a Point

In the preceding discussions, the buffer zones around a point are described as circles. In fact, they are not circles but 23-sided polygons. In most instances, there is little difference, but in some applications this may be significant. An example of the structure of the zone and a zoomed-in portion of the zone are shown in the following illustration.

Fig. 16-6

The actual distance to the buffer edge will vary slightly along the perimeter. The zone in the previous illustration was defined to be 300 km from the point when created. Measurements of the diameter using the measurement tool indicated that the vertices were about 303 km from the center and that the center of the straight line segment was at 300 km.

Because the resulting figure is not a true circle, the area of the feature is not exactly the same as it would be if it were a circle. The area of a 200-km buffer zone is 126,451 km2. If it were a true circle, it would have an area of 200 km x 200 km x 3.14159, which is 125,663 km2. In most map representations of the feature, however, it will appear circular.

Circles and Projections

One possibly confusing factor that can arise when creating buffer zones around points is that the resulting zone boundaries may not look like circles, but like ellipses or other shapes. Buffer zones, like all features, are created in the warehouse to which you directed them. Every warehouse, when created, has a projection (and datum) associated with it. GeoMedia re-projects (if necessary) all information from the warehouse to the current workspace projection, which may be the same or different.

In the process of projection transformations, the shapes or sizes or directions of geographic features may change. The feature has not changed, just its map representation in the new projection. If you have buffer zones in the warehouse and they are not circular on the map window, look at the warehouse projection and the workspace projection. Projections are discussed in detail in Chapter 37 "Projections and Datum Transformations." The following illustration shows three map views of a circular buffer. The workflow that follows takes you through the process of creating a merged buffer zone around point features.

Fig. 16-7

Three map views of a circular buffer created in a UTM warehouse: Cassini-Soldner projection (A), equidistant conic projection (B), and Albers equal area (C).

Workflow 5-1:

Creating Buffer Zones Around Point Features

For this workflow, you need access to the Arkansas.mdb warehouse. In this exercise you will create single merged and unmerged, ring, and stacked buffer zones. To creates these zones, perform the steps that follow.

1. Set up the workspace and warehouse. Start GeoMedia and click on "Create new workspace." If you have the Welcome to GeoMedia dialog turned on, you will have three options: Learning GeoMedia, Open existing GeoWorkspace, or Create new GeoWorkspace. Select Create new GeoWorkspace. If you have elected to dismiss this opening dialog in a previous session, you will not see it and you will, instead, need to select File > New GeoWorkspace from the main menu. From the "New workspace" dialog, select Normal.Gwt and click on OK. Be sure that the Document radio button is selected, notthe Template button. 2. Connect to the Arkansas warehouse. Select Warehouse > New Connection. Select Access and click on Next. Type Arkansas in the "Define connection name" dialog. Click on the browse button and move to the location of the Arkansas.mdb warehouse. Select the Arkansas.mdb warehouse and click on Open.

3. Create a new warehouse in which to place your results. Select Warehouse > New Warehouse. In the "New warehouse" dialog, select Normal.Mdt and click on OK. Name your new warehouse "Buffering" and save it in a directory where you have write privileges.

 When you are done with the exercises for the chapter, you may want to delete the warehouse. 4. Display the data and view the results. Select Legend > Add Feature Class. Select the Arkansas connection and shift click on all features (cities, rivers, interstates, state, and counties).

5. Create 15-km buffers around the cities in the state. Select Insert > Buffer Zones. In the "Buffer zone around" field, select cities from the Arkansas warehouse. Type the warehouse name buffering in the field of the Buffer Zone dialog labeled "Output buffer zone to," and enter City_15km in the "Feature class" field.

 Feature classes cannot start with a number and cannot have spaces or special characters, except for the underscore. 6. Type 15 in the "Constant distance" field, and either type km or select "km" from the drop-down list of measurements in the dialog box to the right of the box in which you entered the numbers. The default measurement value will be whatever has been previously chosen via Tools > Options > Measurement, which is commonly meters. If you forget to enter "km," you will have a 15-m buffer zone and probably will not be able to see it on the map. 7. Select Single from the Buffer Type > Point dialog. Because you selected a point feature to buffer from in the Buffer type dialog area, only the Point dialog option is available. 8. Make sure the Unmerged radio button is selected. Click on OK After a few moments, the new buffer zones will be displayed in the map window, as shown in the following illustration.

Fig. 16-8

If you look carefully in the center of the state, or at the northwest corner, you will see that a number of the single buffers overlap. These features answer the query "Where are the areas that are within 15 km of each city." There are 50 cities and 50 buffer zones in the example. A slightly different query is "Show all areas within 15 km of any city." To answer this type of query, you need to redo the buffering using merged areas.

9. Create 15-km merged buffers around the cities in the state. Select Insert > Buffer Zones. In the "Buffer zone around" field, select "cities" from the Arkansas warehouse. Type the name "buffering" in the "Output buffer zone to" field, and in the "Feature class" field, type City_15km_merged. Feature classes cannot start with a number and cannot have spaces or special characters, except for the underscore. 10. Type 15 in the "Constant distance" field, and either type km or select "km" from the drop-down list of measurements in the dialog field to the right of the box in which you entered the numbers. Do not forget to enter "km;" otherwise, you may have a 15-meter buffer zone and probably will not be able to see it on the map. 11. Select Single from the Buffer Type > Point dialog. Because you selected a point feature to buffer from in the Buffer type dialog area, only the Point dialog option is available. 12. Make sure the Merged radio button is selected. Click on OK. Remove the previous Cities_15km feature class from the map window so that the new results will be easier to see. Highlight the legend entry by clicking on it. Press the Delete key. This time there were only 35 new features added to the warehouse in the feature class Cities_15km_merged. These examples have just considered single zones. Another important type of zone is the multiple zone. This part of the exercise will look at multiple zones around the small town of Hope, Arkansas. 13. Create multiple-ring buffers. Remove the Cities and Cities_15km_merged feature classes from the map window by clicking on them in the legend. Then press Delete. Then press OK. Select Legend > Add Feature Class, and select the Hope feature from the Arkansas warehouse. Use the Zoom button (magnifying glass with a plus) to draw a box around the county Hope lies within. The resulting map window should look something like that shown in the following illustration. Fig. 16-9 14. Create 10-, 15-, and 30-km ring buffers around Hope. Select Insert > Buffer Zones. In the "Buffer zone around" field, select Hope from the Arkansas warehouse. Type buffering in the "Output buffer zone to" field, and in the feature class, type Hope_10_15_30_rings. Feature classes cannot start with a number and cannot have spaces or special characters, except for the underscore. 15. Type 10;15;30 in the "Constant distance" field, and either type km or select "km" from the drop-down list of measurements in the dialog field to the right of the box in which you entered the numbers. Be sure to place a semicolon between the distances. 16. Select Multiple Rings from the Buffer Type > Point dialog. Because you selected a point feature to buffer from in the Buffer type dialog area, only the Point dialog option is available. Because you cannot merge multiple rings (or stacked circles), only the Unmerged radio button is available. 17. Click on OK. You should see a map display similar to that shown in the following illustration.

Fig. 16-10

Each of these is a separate feature with an entry in the warehouse. Use the mouse to select each of the rings. First click on the outside ring. You will see that the outermost circle and the next circle in highlight.

18. Determine areas of ring features. Highlight the outermost ring by clicking on it with the mouse. Click on the Display Select Set Properties button, which is located on the left side of the main GeoMedia window, at the bottom. It is the icon showing a hand holding a sheet of paper. You can also double click on the feature to bring up the Properties dialog. 19. On the Properties dialog, click on the General tab and note the area of each feature so that you can compare the results of the various buffering options. The area of the outermost ring (feature 3) is 2,133.9 km2, the middle ring is 395.2 km2, and the innermost zone is 316.1 km2. The total area within 30 km of Hope is the sum of these or 2,122.9 + 395.2 + 316.1, or a total area of 2,845.2 km2.

If you repeat the process but select Multiple Stacked, the resulting map as displayed will look identical. However, instead of a circle surrounded by two donuts, there are three circles "stacked" on top of one another. If you look at the area of the outer circle, you will see that it is 2,845 km2, the middle circle is 711.3 km2, and the innermost circle is 316.1 km2.
 
 

Creating Buffer Zones
Around Linear Features

There are fewer options for buffering around linear features. There are no options for multiple zones. If you want multiple zones, you must run the buffering operation repeatedly, as is shown in material to follow. You can elect to have the resulting buffer zones be independent features, or merged into a single feature if they overlap.

Simple Unmerged Buffer Zones
Around Linear Features

The following illustrations show the result of a simple unmerged buffer around linear features. The buffers in this case are rounded on the ends.

Fig. 16-11

The preceding illustration on the right shows the same operation as performed in the illustration showing square buffering (previous illustration at left). The option of square or rounded ends provides a useful alternative for different real-world situations. In the rounded case, all areas within the feature boundary are the specified distance from the end of the linear feature. Essentially, a compass needle is placed at the end of the line and a circle inscribed. In the other case, a rectangular corridor is created. The following illustration compares these two alternatives for a simple line.

Fig. 16-3

Merged Buffer Zones
Around Linear Features

The unmerged buffer zone creates a new area feature of the appropriate distance around each selected linear feature. It is often the case that zones from nearby features will overlap. This may be desirable or not. An operation that was designed to answer the question "Provide a list of water meters that are within 10 feet of each of the city’s water mains" would require that each water main have an individual buffer zone. The query "What percentage of the city’s land area is within 10 feet of a water main" would require that the overlapping zones be merged. Examples of merged round and square buffers, respectively, are shown in the following illustrations.

Fig. 16-4

Line Segments and Buffering

An important factor to consider when buffering linear features is the various ways in which linear objects in the world can be represented in the GeoMedia data model. Consider a highway through an area. The highway from start to finish is "Road 71." It may be represented as a single linear feature from beginning to end, with many vertices along the way that represent turns in the road. In this representation, the entire road is stored in a GeoMedia warehouse as a single feature instance.

It is also possible to represent the road as a set of road segments, with a different road segment connecting each vertex along the road, or with small groups of vertices combined into one segment. In this case, the road will be represented by a number of feature instances in the warehouse or record set.

The illustration at left shows two lines. The bottom line is one feature, and the upper line consists of four different features. The bottom line has a single record in the warehouse and the upper one has four.

Although these two linear objects look similar, they will have very different buffer zones created from them. In the case of the upper line, there are four buffer zones created, whereas for the lower line there is only one. The illustration at left shows these line conditions.

If you have a set of connected line segments (such as in the previous illustration) and you want a single buffer zone to encompass all of them, you will need to use the Merge option. In most situations you will need to create a query set of all line segment features you want in the single buffer zone. Any query set where the road name is "Road 71" would select only those feature instances that represented this road. (See Part V for details on how to create a query set.) Once created, you would enter the name of this query set as the feature to buffer from.

There is no multiple ring or stacked option available for linear features. There are workarounds, however, that will allow you to create stacked multiple distance zones around a liner feature. These workarounds are discussed later in the chapter.

Creating Buffer Zones
Around Area Features

Creating buffer zones around area features is basically quite simple, but there are some important points to remember. As in the case of linear features, there are no options for multiple rings or stacked areas. On the other hand, area buffering can take place inside or outside the area and, as was the case with linear features, the results can be either merged or unmerged. The results of a simple unmerged buffering are shown in the illustration at left.

When an area feature is buffered using the outside option, the buffer zone includes the area. The buffering distance is added to the feature and a larger feature is produced. In the previous illustration, the buffer zones are not the rings around the areas but the entire surface, including what was once the area from which the buffering occurred. They look like rings in this example because they are shown with transparent fill. If the fill for the buffer zone were solid, the outer zone feature would cover the inner one.

Suppose you were interested in the area outside the city but within 5 miles of the city limits. The buffering operator would be able to create an area feature that covered the 5 miles outside the city and that included the city proper. It would be very easy to create a map that used one color for the city and one for the outside area. However, in GeoMedia you would not be able to create a feature that represented just the area from the city limits out 5 miles. However, in the GeoMedia buffering operator you would not be able to create a feature that represented just the area from the city limits out 5 miles. To do this you will need to take the resulting feature (which includes both the edge to five mile area and the city area as one feature, and use the Spatial Difference operation that allows you to "cut" parts out of one feature using another. In this case you would "cut" the larger, combined feature, with the city limit feature leaving only the "ring" from the city limit out to 5 miles. See Chapter 18 "spatial Overlay" for details on how to use the Spatial Difference operator. When unmerged zones are created, there is one zone for each area feature involved. In the example shown in the previous illustration, there are three buffer zone features.

Creating Merged Outside Buffer Zones
 

As was the case for point and linear features, there is a need for merged zones from area features. The illustration at left shows the same buffer zone used in the previous illustration. However, this time the resulting features are merged if they spatially overlap. There are only two area buffer zone features shown in the illustration at left. The large one in the southwest encompasses all of the area of the two nearest initial starting features and the resulting buffer zones.

Using the Inside Option
to Buffer Area Features

It is also possible to buffer inside an area feature. The following illustration shows inside buffering applied to the same area features used in the previous examples.

Fig. 16-21

Looking at the vector representation of the results of inside buffering, it might be assumed that the donut just within the edge of the original feature would be the resulting output buffer zone, but this is not the case. The actual output feature from the operation is the "hole" of the donut. For example, the following illustration at right incorporates the same features as the illustration at left, but here the resulting buffer zone feature has been cross hatched.

Consider the inside area buffer operation as determining the portion of the area feature inside the buffer zone distance. This buffering option would be used to answer queries such as "Identify all areas within the city limits but more than 1 km inside the boundary" or "Create a map of the lake that shows all areas 200 yards from the shore." In the previous illustration, three such zones were created, and three records created in the warehouse.

Creating Multiple Buffer Rings
Around Linear and Area Features

There is no multiple distance option for linear or area features in GeoMedia, as there is for point features. It is quite easy to create these multiple zones, however. Suppose you wanted a map that showed the area within 10 km and within 20 km of a road. The workflow is to initially create the 10 km buffer zone around the road, and then to create a second area buffer zone around the first.

In the first operation, the buffered feature would be a linear one (the road), and the output would be an area feature (road_10km_buffer). However, the second operation on road_10km_buffer would be a buffering operation on an area feature because the output of the first buffering created an area feature. This process could be repeated as many times as necessary.

Similar operations could create buffer zones around an area feature; for example, zones of 1 km and 5 km around a city. In both the linear and area examples, the resulting buffer zones are equivalent to the point stacked rings. Each outer buffer zone encompasses all the area of the inner one (or of the area from which it was initiated). It is not possible to create concentric ring buffer zones around linear or area features in GeoMedia. The illustration at left shows the results of the creation of two stacked buffer zones around a single linear feature.