Cartographic generalization

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Generalization has a long history in cartography as an art of creating maps for different scale and purpose. Cartographic generalization is the process of selecting and representing information of a map in a way that adapts to the scale of the display medium of the map. In this way, every map has, to some extent, been generalized to match the criteria of display. This includes small-scale maps, which cannot convey every detail of the real world.

Cartographers must decide and then adjust the content within their maps to create a suitable and useful map that conveys geospatial information within their representation of the world.

Generalization is meant to be context specific. This is to say that correctly generalized maps are those that emphasize the most important map elements while still representing the world in the most faithful and recognizable way. The level of detail and importance in what is remaining on the map must outweigh the insignificance of items that were generalized, as to preserve the distinguishing characteristics of what makes the map useful and important.

Contents 1 Selection 2 Simplification 3 Combination 4 Merge (Amalgamation) 5 Exaggerate 6 Aggregation 7 Collapse 8 Refinement 9 Typify 10 Displacement 11 Smoothing 12 Enhancement 13 Adjust Color 14 Eliminate 15 Reclassify 16 Rotate operator 17 Operators in automated generalization 18 Add Operator 19 References

[edit] Selection

Map generalization can take many forms, and is designed to reduce the complexities of the real world by strategically reducing ancillary and unnecessary details. One way that geospatial data can be reduced is through the selection process. The cartographer can select and retain certain elements that he/she deems the most necessary or appropriate. In this method, the most important elements stand out while lesser elements are left out entirely. For example, a directional map between two points may have lesser and un-traveled roadways omitted as not to confuse the map-reader. The selection of the most direct and uncomplicated route between the two points is the most important data, and the cartographer may choose to emphasize this.

[edit] Simplification

Generalization is not a process that only removes and selects data, but also a process that simplifies it as well. Simplification is a technique where shapes of retained features are altered to enhance visibility and reduce complexity. Smaller scale maps have more simplified features than larger scale maps because they simply exhibit more area. An example of simplification is to scale and remove points along an area. Doing this to a mountain would reduce the detail in and around the mountain but would ideally not detract from the map reader interpreting the feature as such a mountain.

[edit] Combination

Simplification also takes on other roles when considering the role of combination. Overall data reduction techniques can also mean that in addition to generalizing elements of particular features, features can also be combined when their separation is irrelevant to the map focus. A mountain chain may be isolated into several smaller ridges and peaks with intermittent forest in the natural environment, but shown as a contiguous chain on the map, as determined by scale. The map reader has to, again remember, that because of scale limitations combined elements are not concise depictions of natural or manmade features.

[edit] Merge (Amalgamation)

Amalgamation and merging take multiple objects and combine them into a single feature to generalize the map. Amalgamation deals with joining of polygons^[1], while merge deals with joining of lines. Amalgamation and merging are necessary with a reduction in scale or scope, because the features become harder to distinguish as they are clustered closer together on a map. There are two types of amalgamation: noncontinuous and continuous. In noncontinuous amalgamation, features that are too small to be seen are grouped together to be represented. An example of noncontinuous amalgamation is bunching the buildings of a school together and representing them with a single building. Continuous amalgamation occurs when features of similar attributes are grouped together.^[1] Merging takes multiple lines and joins them, often finding the average placement and using that for the new line.^[1]

[edit] Exaggerate

The Exaggerate opperator is a way of generalizing objects on a map in a way that makes them stand out more than they normally would without the exageration. An example of this is to make a small slender spit of land larger and/or thiker in order to see its charactoristics more clearly in large scale maps. This opperater is often used to show subtle differences more clearly. Often times when making a 3 dimensional rendering of a mountain or other topographical feature the Z dimension (Z dimension = altitude or height) will be exagerated to show more clearly the relationship of the terrain with nearby area. The exaggerate opperator is most often used to exaggerate only the portion that needs to be viewed more clearly. In the example of the spit of land, only the spit would be exaggerated in its geometry leaving the rest of the land in its original geometry. In the example of exaggerating the mountain peaks, the Z dimension is exaggerated , but the X and Y dimensions are left the same.

[edit] Aggregation

Aggregation is a method of map generalization which combines features of similar characteristics. As a method of reclassification, aggregation reduces the number of classes.^[2] Aggregate data is used for summarization, partitioning, simplification, and confidentiality purposes. For example, aggregation is used in presenting census data, where individual privacy is paramount.^[3] Two challenges exist for one wishing to aggregate data. One is knowing how homogenous or densely-spaced data must be in order to be combined in one feature, and the other is deciding the boundary of the aggregate area.^[1]

As a tool for generalizing a map, aggregation can inhibit data analysis at the original, unsimplified level. Detail found on the original level is agglomerated into one feature, obscuring the properties of the various components.

[edit] Collapse

The collapse operation involves the conversion of geometry, such the simplification of a polygon to a line or a line to a point. This allows for an otherwise complex feature, such as an urban area, to be reduced to a point that is resymbolized with a geometric form like a circle or square that has a different (often smaller) size. In cartography this operator helps to de-clutter a map, especially when there is an increase in scale, making details harder to see. This only applies when the object only serves as a geographical reference. Otherwise, the detail being removed from the object would change the purpose of the map.

In relation to other operators, collapse performs the opposite function as aggregate and differs from adjust shape due to its change is symbol size.

[edit] Refinement

Refinement is another form of re-symbolization similar to collapse, though it is an operation that involves reducing a multiple set of features such as roads, buildings and other urban structures to a simplified representation, rather than a conversion to a geometric shape.

[edit] Typify

Also known as “distribution refinement,” the typify operator is the replacement of a large, often cluttered set of similar features, with a smaller set of those features[1]. The typify operator can be used to simplify a distribution of points, lines and polygons, but must also represent the spatial density of the objects with fewer. This reduction cannot take place in a random fashion, but must take into account the spatial distribution of the objects being reduced. The typify operator is distinguished from the eliminate operator, which simplifies the map by removing some of the original features in a group, by replacing them with a new arrangement of fewer features. A common use of the typify operator is the use of two or three line symbols to represent a river delta, rather than the use of numerous lines to represent all of the streams in the delta.

Another example of typify is the buildings in a small scale map. The buildings cannot be represented individually, but the spatial reference that the buildings occupy can be symbolized in fewer symbols than individual buildings while covering the same area that individual buildings occupy. This is very common when maps are made into smaller scales.

[edit] Displacement

To displace a feature means to alter its absolute location to preserve its unique identity^[4] Displacing a feature may be an effective cartographic choice when faced with a few issues. One may want to move a feature slightly out of the way of another to allow both features to be clearly distinguished by the map viewer. For example, in the first map on the left of Prince George's County, Maryland, the three main arterial routes to Washington, D.C. - I-95, U.S. Route 1, and the Baltimore-Washington Parkway - are difficult to distinguish in their true locations. However, when the features are slightly moved, as in the second map, the three routes can be more easily distinguish. While this technique compromises the accuracy of absolute location, it clarifies relative location. Therefore, this generalization technique should be used with caution.

The displace operator makes it possible to adjust the location of features to avoid overlapping at certain scales. While two objects may have negative space separating them at a large (fine) scale, this space could be reduced to nothing at a smaller scale, making the symbols difficult to discern. The displace operator can keep the symbols/features separate from each other by increasing the distance between them when zooming out. This can help manage the problems that arise from displaying different features that are close to each other, even at various scales.

[edit] Smoothing

Smoothing is also a process that the map maker can employ to reduce the angularity of line work. Smoothing is yet another way of simplifying the map features, but involves several other characteristics of generalization that lead into feature displacement and locational shifting. The purpose of smoothing is exhibit linework in a much less complicated and a less visually jarring way. An example of smoothing would be for a jagged roadway, cut through a mountain, to be smoothed out so that the angular turns and transitions appear much more fluid and natural.

[edit] Enhancement

Enhancement is also a method that can be employed by the cartographer to illuminate specific elements that aid in map reading. As many of the aforementioned generalizing methods focus on the reduction and omission of detail, the enhancement method concentrates on the addition of detail. Enhancement can be used to describe the true character of the feature being represented and is often used by the cartographer to highlight specific details about his or her specific knowledge, that would otherwise be left out. An example includes enhancing the detail about specific river rapids so that the map reader may know the facets of traversing the most difficult sections beforehand. Enhancement can be a valuable tool in aiding the map reader to elements that carry significant weight to the map’s intent.

[edit] Adjust Color

The adjust color operator alters the hue, value, or saturation of a feature so that it remains legible across multiple scales. A change in scale may adjust the color distribution on the map enough to produce situations of simultaneous contrast and color illegibility not present in larger scale versions. Therefore, the adjust color operator may be implemented for two reasons: (1) to increase the position of a feature in the visual hierarchy by increasing its contrast or distinctiveness or (2) to increase the position of surrounding features in the visual hierarchy by decreasing the resymbolized feature’s contrast or distinctiveness.

[edit] Eliminate

The eliminate operator can be implemented to remove features when they become unnecessary or illegible at a certain scale. When objects don't portray a clear message or fulfill the purpose they were created for it may be useful to use this operator. There are four conditions that generally indicate when the eliminate operator should be used. If:

• the data has too detailed of a resolution for the viewing scale that causes a mismatch with other layers.
• the data has too detailed of a resolution, thus providing unnecessary information.
• there are too many layers at a given scale which creates illegibility.
• only the most significant features in a grouping are required to convey a specific message.

There is also a special case that has been identified by researchers in which a subset of features is eliminated from a larger whole to help reinforce the order of the visual hierarchy.^[4]

The figure below shows an example of elimination. In the picture with a larger scale to the left you can see the smaller streets but on the right where the scale is smaller the smaller streets could not be shown legibly so they have been removed.

[edit] Reclassify

During the reclassification process, the organization of features are changed based on attributes in order to improve legibility within the map. This can be accomplished in one of three ways:

• A revision to the total number of classes represented
• A revision to the composition of existing classes
• A combination of both of the above

Reclassifying more specifically deals with taking input cell values and replacing them with new output cell values. This will in many instances do as was mentioned above; simplify or change the interpretation of raster data by changing a single value to a new value. ^[5]

[edit] Rotate operator

The rotate feature will change the orientation of one feature in relation to other features. Rotation is the 360 degree shift in orientation of an object. The rotate operator differs from other operatorssuch as the displace operator in that it will change orientation where the displace operator only changes location. It also differs from the exagerate operator because it will move the entire object and not just parts of it. One of the many uses for the rotate operator is to align buildings with roads after they have been collapsed.^[4]

[edit] Operators in automated generalization

Automated generalization had always to compete with manual cartographers, therefore the manual generalization process was studied intensively. These studies resulted early in different generalization operators. By now there is no clear classification of operators available and it is doubtful if a comprehensive classification will evolve in future.

[edit] Add Operator

The add operator inserts new features to the map display that are only appropriate for representation at smaller scales. Such layers may be useless, and even deceiving, at large scales, but can be included in the representation once the scale has been reduced to a sufficient level. This technique can be used in both Cartography and Remote Sensing.

[edit] References

1. ↑ ^{1.0 1.1 1.2 1.3} Slocum, Terry A., McMaster, Robert B., Kessler, Fritz C., Howard, Hugh H. 1999. Thematic Cartography and Visualization. Prentice Hall, Upper Saddle River, NJ. p.p.103
2. ↑ Demers, Michael N. Fundamentals of Geographic Information Systems. 4th ed. John Wiley and sons, inc. 2009. p. 231.
3. ↑ Rao, Anamika S. What Do You Mean by GIS Aggregation? Publish Your Articles. PublishYourArticles.org, n.d. Web. 6 September 2011.
4. ↑ ^{4.0 4.1 4.2} Roth, R., Stryker, M., & Brewer, C. A. The ScaleMaster Typology: Literature Foundation. Retrieved 9 Oct. 2011.
5. ↑ "Reclassification - GIS Dictionary." Esri Support. Web. 10 Oct. 2011. http://support.esri.com/en/knowledgebase/GISDictionary/term/reclassification

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