GIS analysis functions use the spatial and non-spatial attribute datato answer questions about real-world. It is the spatial analysis functionsthat distinguishes GIS from other information systems.
When use GIS to address real-world problems, you'll come up against thequestion that which analysis function you want to use and to solve the problems.In this case, you should be aware that wisely using functions will leadto high quality of the information produced from GIS and individual analysisfunctions must be used in the context of a complete analysis strategy. (StanAronoff, 1989)
Spatial data refers to information about the location and shape of, andrelationships among, geographic features, usually stored as coordinatesand topology. Spatial data functions are used to transform spatial datafiles, such as digitized map, edit them, and assess their accuracy. Theyare mainly concerned with the spatial data.
Format is the pattern into which data are systematically arranged foruse on a computer. Format transformations are used to get data into acceptableGIS format. Digital Files must be transformed into the data format usedby the GIS, such as transforming from raster to vector data structure. Araster data often requires no re-formatting. A vector data often requirestopology to be built from coordinate data, such as arc/node translations.Transformation can be very costly and time-consuming with poor coordinatedata.
Geometric transformations are used to assign ground coordinates to amap or data layer within the GIS or to adjust one data layer so it can becorrectly overlayed on another of the same area. The procedure used to accomplishthis correction is termed registration.
Two approaches are used in registration: the adjustment of absolute positionsand the adjustment of relative position. Relative Position refersto the location of features in relation to a geographic coordinate system.Rubber sheeting (registration by Relative Position) is the procedureusing "slave" and "master" mathematical transformationsto adjust coverage features in a nonuniform manner. Links representing from-and to-locations are used to define the adjustment. It needs easily identifiable,accurate, well distributed control points. Absolute Position is thelocation in relation to the ground. This registration is done by individuallayers. The advantage is that it does not propagate errors.
Map projection is a mathematical transformation that is used to representa spherical surface on a flat map. The transformation assigns to each locationon a spherical surface a unique location on a 2-dimensional map.
Map projections always causes some distortion: area, shape, distance,or direction distortion. GIS commonly supports several projections and hassoftware to transform data from one projection to another. The map projectionsmost commonly used for mapping at scales of 1:500,000 or larger in NorthAmerica is the UTM(Universal Transverse Mercator) Projection. For maps ofcontinental extent, the Albers, Lambert's Azimuthal, and Polyconic projectionsare commonly used.
Conflation is the procedure of reconciling the positions of correspondingfeatures in different data layers. Conflation functions are used to reconcilethese differences so that the corresponding features overlay precisely.This is important when data from several data layers are used in an analysis.
Edge matching is a procedure to adjust the position of features extendingacross map sheet boundaries. This function ensures that all features thatcross adjacent map sheets have the same edge locations. Links are used whenmatching features in adjacent coverages.
Editing functions are used to add, delete, and change the geographicposition of features. Sliver or splinter polygons are thinpolygons that are occurring along the borders of polygons following digitizingand the topological overlay of two or more coverages.
Address Matching is a mechanism for relating two files using addressas the relate item. Geographic coordinates and attributes can be transferredfrom one address to the other. For example, a data file containing studentaddresses can be matched to a street coverage that contains addresses creatinga point coverage of where the students live.
The Thinning function reviews all the coordinate data in a file, identifiesand then removes unnecessary coordinates. Depending on scale, a number ofcoordinate pairs can often be significantly reduced without a perceivedloss of detail
This function is used to reduce the quantity of coordinate data thatmust be stored by the GIS. Coordinate thinning, by reducing the number ofcoordinate points, reduces the size of the data file, thereby reducing thevolume of data to be stored and processed in the GIS.
Attribute Data is relate to the description of the map items. It is typicallystored in tabular format and linked to the feature by a user-assigned identifier(e.g., the attributes of a well might include depth and gallons per minute).
Retrieval operations on the spatial and attribute data involve the selectivesearch manipulation, and output of data without the need to modify the geographiclocation of features or to create new spatial entities. These operationswork with the spatial elements as they were entered in the data base.
Information from database tables can be accessed directly through themap, or new maps can be created using information in the tabular database.Both graphic and tabular data must be stored in formats the computer canrecognize and retrieve.
Classification is the procedure of identifying a set of features as belongingto a group and defining patterns. Some form of classification function isprovided in every GIS. In a raster-based GIS, numerical values are oftenused to indicate classes. Classification is important because it definespatterns. One of the important functions of a GIS is to assist in recognizingnew patterns.
Classification is done using single data layers, as well as with multipledata layers as part of an overlay operation.
Generalization, also called map dissolve, is the process ofmaking a classification less detailed by combining classes. Generalizationis often used to reduce the level of classification detail to make an underlyingpattern more apparent.
Verification is a procedure for checking the values of attributes forall records in a database against their correct values. (Keith C. Clarke,1997)
Overlay is a GIS operation in which layers with a common, registeredmap base are joined on the basis of their occupation of space. (Keith C.Clarke, 1997).
The overlay function creates composite maps by combining diverse datasets. The overlay function can perform simple operations such as layinga road map over a map of local wetlands, or more sophisticated operationssuch as multiplying and adding map attributes of different value to determineaverages and co-occurrences.
Raster and vector models differ significantly in the way overlay operationsare implemented. Overlay operations are usually performed more efficientlyin raster-based systems. In many GISs a hybrid approach is used that takesadvantage of the capabilities of both data models. A vector-based systemmay implement some functions in the raster domain by performing a vector-to-rasterconversion on the input data, doing the processing as a raster operation,and converting the raster result back to a vector file.
Region Wide Overlay: "Cookie Cutter Approach"
The region wide, or "cookie cutter," approach to overlay analysisallows natural features, such as forest stand boundaries or soil polygons,to become the spatial area(s) which will be analyzed on another map.
For example ( see figures above): given two data sets, forest patchesand slope, what is the area-weighted average slope within each separatepatch of forest? To answer this question, the GIS overlays each patch offorest from the forest patch data set onto the slope map and then calculatesthe area-weighted average slope for each individual forest patch.
Topological Overlay:
Co-Occurrence mapping in a vector GIS is accomplished by topologicaloverlaying. Any number of maps may be overlayed to show features occurringat the same location. To accomplish this, the GIS first stacks maps on topof one another and finds all new intersecting lines. Second, new nodes (pointfeatures where three or more arcs, or lines, come together) are set at thesenew intersections. Lastly, the topologic structure of the data is rebuiltand the multifactor attributes are attached to the new area features.
Neighborhood Function analyzes the relationship between an object andsimilar surrounding objects. For example, in a certain area, analysis ofa kind of land use is next to what kinds of land use can be done by usingthis function. This type of analysis is often used in image processing.A new map is created by computing the value assigned to a location as afunction of the independent values surrounding that location. Neighborhoodfunctions are particularly valuable in evaluating the character of a localarea.
Point-in-Polygon is a topological overlay procedure which determinesthe spatial coincidence of points and polygons. Points are assigned theattributes of the polygons within which they fall. For example, this functioncan be used to analyze an address and find out if it (point) is locatedwithin a certain zip code area (polygon).
Line-in-Polygon is a spatial operation in which lines in one coverageare overlaid with polygons of another coverage to determine which lines,or portions of lines, are contained within the polygons. Polygon attributesare associated with corresponding lines in the resulting line coverage.For example, this function can be used to find out who will be affectedwhen putting in a new powerline in an area.
In a vector-based GIS, the identification of points and lines containedwithin a polygon area is a specialized search function. In a raster-basedGIS, it is essentially an overlay operation, with the polygons in one datalayer and the points and/or lines in a second data layer.
Topography refers to the surface characteristics with continuously changingvalue over an area such as elevations, aeromagnetics, noise levels, incomelevels, and pollution levels. The topography of a land surface can be representedin a GIS by digital elevation data. An alternative form of representationis the Triangulated Irregular Network or TIN used in vector-based systems.
Topographic functions are used to calculate values that describe the topographyat a specific geographic location or in the vicinity of the location. Thetwo most commonly used terrain parameters are the slope and aspect,which are calculated using the elevation data of the neighbouring points.
Slope is the measure of change in surface value over distance, expressedin degrees or as a percentage. For example, a rise of 2 meters over a distanceof 100 meters describes a 2% slope with an angle of 1.15. Mathematically,slope is referred to as the first derivative of the surface. The maximumslope is termed the gradient. In a raster format DEM, another gridwhere each cell is the slope at a certain position can be created, thenthe maximun difference can be found and the gradient can be determined.Aspect is the direction that a surface faces. Aspect is defined bythe horizontal and vertical angles that the surface faces. In a raster formatDEM, another grid can be created for aspect and a number can be assignedto a specific direction.
Sun intensity is the combination of slope and aspect. Illuminationportrays the effect of shining a light onto a 3-dimensional surface. (StanAronoff, 1989).
Thiessen or voronoi polygons define individual areas of influence aroundeach of a set of points. Thiessen polygons are polygons whose boundariesdefine the area that is closest to each point relative to all other points.Thiessen polygons are generated from a set of points. They are mathematicallydefined by the perpendicular bisectors of the lines between all points.A tin structure is used to create Thiessen polygons.
Interpolation is the procedure of predicting unknown values using theknown values at neighboring locations. The quality of the interpolationresults depends on the accuracy, number, and distribution of the known pointsused in the calculation and on how well the mathematical function correctlymodels the phenomenon.
4. Cartographic Modeling
5. Connectivity Functions
6. Output Functions
1. conflation 2. edge-matching 3. line coordinate thinning 4. addressmatching 5. rubber sheeting 6. retrieval 7. classification 8. overlay 9.point-in-polygon 10. thiessen polygon 11. interpolation 12. line-in-polygon
1. Please present your own opinion on how to use GIS analysis functions.
2. Please discuss "Spatial analysis functions are the power of GIS"?
3. Please give example to describe GIS overlay function.
4. Describe differences and relationships of spatial data and attributedata.
5. Using figures or examples to describe Edge Matching Function.
6. What is Sliver Polygon? Describe how to create sliver polygons andusing what function to delete them.
7. Describe what are slope and aspect, how to measure them.
Note: please see Michael L. Hauschild's page forthe last three parts.
Submitted by Chengdai Liu.