Lecture11_VECTOR DATA ANALYSIS.pdf

NGEO624 Introduction to GIS The science of Where, What, When, and Why? Lecture 11 (GEOSPATIAL ANALYSIS I: VECTOR DATA ANALYSIS) Maxwell Djeco (Office. No. 316) [email protected] School of Natural and App...

NGEO624 Introduction to GIS The science of Where, What, When, and Why? Lecture 11 (GEOSPATIAL ANALYSIS I: VECTOR DATA ANALYSIS) Maxwell Djeco (Office. No. 316) [email protected] School of Natural and Applied Sciences  The scope of analyses using a geographic information system (GIS) varies among disciplines. GIS users in hydrology will likely emphasize the importance of terrain analysis and hydrologic modelling, where as GIS users in wildlife management will be more interested in analytical functions dealing with wildlife point locations and their relationship to the environment. This is why GIS developers have taken two general approaches to packaging their products. One prepares a set of basic tools used by most GIS users, and the other prepares extensions designed for specific applications such as hydrologic modelling.  The vector data model uses points and their x-, and y-coordinates to construct spatial features of points, lines, and polygons. These spatial features are used as inputs in vector data analysis. The accuracy of data analysis depends on the accuracy of these features in terms of their location and shape and whether they are topological or not.  The vector data analysis can include: Buffering Overlay Measuring distances Pattern analysis Feature manipulation Tools.  BUFFERING Based on the concept of proximity, buffering creates two areas: one area that is within a specified distance of select features and the other area that is beyond. The area within the specified distance is the buffer zone.  The buffer distance or buffer size does not have to be constant. It can vary according to the values of a given field. For example, stream buffer sizes may vary depending on the intensity of adjacent land use. Variations in A map feature may have more than one buffer zone. For example, a nuclear power plant may be buffered with distances of 5, 10, 15, and 20 Buffering miles, thus forming multiple rings around the plant. Boundaries of buffer zones may remain intact so that each buffer zone is a separate polygon. These boundaries may be dissolved so that there are no overlapped areas between buffer zones. Buffering with different buffer distances  The buffer distance or buffer size does not have to be constant; it can vary according to the values of a given field. For example, the width of the riparian buffer can vary depending on its expected function and the intensity of adjacent land use. A feature may have more than one buffer zone. As an example, a nuclear power plant may be buffered with distances of 5, 10, Buffer zones dissolved Buffering with four rings 15, and 20 km, thus forming (top) or not dissolved multiple rings around the plant. (bottom)  Riparian buffers are strips of land along the banks of rivers and streams that can filter polluted runoff and provide a transition zone between water and human land use. Riparian buffers are also complex ecosystems that can protect wildlife habitat and fisheries. Depending on what the buffer is supposed to protect or provide, the buffer width can vary.  Applications of Buffering A buffer zone is often treated as a protection zone for planning or regulatory purposes. A city ordinance may stipulate that no liquor stores shops shall be within 1000 m of a school or a church. Government regulations may stipulate that logging operations must be at least 2 km away from any stream to minimize the sedimentation problem and set the 2 km buffer zones of streams as the exclusion zones. A national forest may restrict oil and gas well drilling within 500 m of roads or highways; 200 m of trails; 500 m of streams, lakes, ponds, or reservoirs; or 400 m of springs. A urban planning agency may set aside land along the edges of streams to reduce the effects of nutrient, sediment, and pesticide runoff; to maintain shade to prevent the rise of stream temperature; and to provide shelter for wildlife and aquatic life. A resource agency may establish stream buffers or vegetated filter strips to protect aquatic resources from adjacent agricultural land use practices.  A buffer zone may be treated as a neutral zone and as a tool for conflict resolution. In controlling the protesting mass, police may require protesters to be at least 300 m from a building. Perhaps the best-known neutral zone is the demilitarized zone separating North Korea from South Korea along the 38° N parallel.  OVERLAY An overlay operation combines the geometries and attributes of two feature layers to create the output. (A GIS package may offer overlay operations with more than two layers at a time; this chapter limits the discussion to two layers for the purpose of clarity.) The geometry of the output represents the geometric intersection of features from the input layers. Feature Type and Overlay In practice, the first consideration for the overlay is feature type. Overlay operations can take polygon, line, or point layers as the inputs and create an output of a lower-dimension feature type. For example, given the inputs of polygon and line layers, the output will be a line layer. Point-in-Polygon overlay: operation, the same point features in the input layer are included in the output but each point is assigned with attributes of the polygon within which it falls. Line-in-Polygon Overlay operation, the output contains the same line features as in the input layer but each line feature is dissected by the polygon boundaries on the overlay layer Polygon-on-Polygon, (the most common overlay operation) is involving two polygon layers. The output combines the polygon boundaries from the input and overlay layers to create a new set of polygons. Overlay Methods Union preserves all features from the inputs. The area extent of the output combines the area extent of both input layers. Intersect preserves only those features that fall within the area extent common to the inputs Symmetrical Difference preserves features that fall within the area extent that is common to only one of the inputs  Identity preserves only features that fall within the area extent of the layer defined as the input layer. The other layer is called the identity layer. Slivers A common error from overlaying polygon layers is slivers, very small polygons along correlated or shared boundary lines (e.g., the study area boundary) of the input layers. The existence of slivers often results from digitizing errors. Because of the high precision of manual digitizing or scanning, the shared boundaries on the input layers are rarely on top of one another. Applications of Overlay The overlay methods play a central role in many querying and modelling applications. Suppose an investment company is looking for a land parcel that is zoned commercial, not subject to flooding, and not more than 1 km from a heavy- duty road. The company can first create the 1-mile road buffer and overlay the buffer zone layer with the zoning and floodplain layers. A subsequent query of the overlay output can select land parcels that satisfy the company’s selection criteria. DISTANCE MEASUREMENT Distance measurement refers to measuring straight-line (Euclidean) distances between features. Measurements can be made from points in a layer to points in another layer, or from each point in a layer to its nearest point or line in another layer.  PATTERN ANALYSIS Pattern analysis is the study of the spatial arrangements of point or polygon features in two-dimensional space. Pattern analysis uses distance measurements as inputs and statistics (spatial statistics) for describing the distribution pattern. Analysis of Random and Nonrandom Patterns A classic technique for point pattern analysis, the nearest neighbour analysis uses the distance between each point and its closest neighbouring point in a layer to determine if the point pattern is random, regular, or clustered. Observed average distance Hypothetical random distribution Applications of Pattern Analysis Pattern analysis has many applications. The nearest neighbour analysis is a standard method for analyzing the spatial distribution and structure of plant species. Hot spot analysis is a standard tool for mapping and analyzing crime locations and public health data. A study of drug hot spots. Spatial autocorrelation in useful for analysing temporal changes of spatial distributions. Likewise, it is useful for quantifying the spatial dependency over distance classes. Spatial autocorrelation is also important for validating the use of standard statistical analysis such as regression analysis  FEATURE MANIPULATION Tools are available in a GIS package for manipulating and managing features in one or more feature layers. When a tool involves two layers, the layers must be based on the same coordinate system. Like overlay, these feature tools are often needed for data preprocessing and data analysis; however, unlike overlay, these tools do not combine geometries and attributes from input layers into a single layer. Feature manipulation is easy to follow graphically, even though terms describing the various tools may differ between GIS packages. Dissolve aggregates features in a feature layer that have the same attribute value or values. For example, we can aggregate roads by highway number or counties by state. An important application of Dissolve is to simplify a classified polygon layer. Clip creates a new layer that includes only those features of the input layer, including their attributes, that fall within the area extent of the clip layer. Clip is a useful tool, for example, for cutting a map acquired elsewhere to fit a study area. The input may be a point, line, or polygon layer, but the clip layer must be a polygon layer. The output has the same feature type as the input. Append creates a new layer by piecing together two or more layers, which represent the same feature and have the same attributes Select creates a new layer that contains features selected from a user-defined query expression Eliminate creates a new layer by removing features that meet a user-defined query expression Update uses a “cut and paste” operation to replace the input layer with the update layer and its features. Erase removes from the input layer those features that fall within the area extent of the erase layer. Split divides the input layer into two or more layers. A split layer, which shows area subunits, is used as the template for dividing the input layer.