Geog 380: Geospatial Communication - Vector Analysis PDF
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2023
Geoffrey Hay
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Summary
This document is a lecture on geospatial communication, specifically focusing on spatial analysis using vectors. It covers learning outcomes, processing differences between raster and vector data, problems with data conversion, choosing between data models, conversions between raster and vector, and GIS analysis terminology.
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Geog 380: Geospatial Communication Source: https://earth.nullschool.net Topic 11: Spatial Analysis - Vectors GEOG 380 - Topic 11 © Geoffrey Hay (2023) 1 Learning outcomes By the end of this lecture topic and associated labs, a successful student will be able to: § Recognize some of the option...
Geog 380: Geospatial Communication Source: https://earth.nullschool.net Topic 11: Spatial Analysis - Vectors GEOG 380 - Topic 11 © Geoffrey Hay (2023) 1 Learning outcomes By the end of this lecture topic and associated labs, a successful student will be able to: § Recognize some of the options and strategies for three major categories of GIS analysis operations: (i) queries, (ii) measurement, and (iii) transformations § Perform some simple overlay and buffering operations GEOG 380 - Topic 11 2 Processing in Raster & Vector § New geospatial technicians often think of raster and vector data analysis as two different worlds § In reality, most software packages provide relatively seamless integration between the two (Spatial Analyst in ArcGIS Pro) § Which is better? http://gsp.humboldt.edu/olm/Lessons/GIS/08%20Rasters/ RasterToVector.html GEOG 380 - Topic 11 § Actually, most operations can be accomplished with both data models § However, conversion between the two can introduce errors § Often best to stick with the “native” data format 3 Problems with Conversion § Whenever you convert data from raster to vector or vector to raster, you will degrade the data by some amount. Issues include: https://vimeo.com/53016715 § § § § GEOG 380 - Topic 11 Loss of detail Loss of accuracy Stair stepping (raster to vector) Changes to the original data 4 Choosing Between Data Models “Raster is faster but vector is corrector” § Raster data is useful when: § Working with continuous data types: elevation, slope, satellite photos § Good for large area analyses § Good for surface analysis § Mathematical modeling § Spatial detail isn't important GEOG 380 - Topic 11 § Vector data is useful when: § Working with discrete data types: trees, buildings, property boundaries § Good for small study areas § Spatial detail is important (When "close enough" isn't really good enough) § When topology is needed for the analysis 5 Choosing Between Data Models § Other factors that can influence your decision include: § Available storage: some rasters are really large (think Petabytes = 1000 TB) § “Some estimates hold that a Petabyte is the equivalent of 20 million tall filing cabinets or 500 billion pages of standard printed text” - Google § Expected types of analysis: some tools only work with raster or vector data § Expertise of human operators § Level of accuracy desired GEOG 380 - Topic 11 6 Conversions § Raster to Points: § Raster to Point § Raster to Polyline: https://www.onestopgis.com/GIS-Theory-and-Techniques/Raster-DataAnalysis/Comparison-of-Vector-and-Raster-based-Data-Analysis/1Comparison-of-Vector-and-Raster-Based-Data-Analysis.html § § § § Contour Stream to Feature Raster to polyline Cost Path § Raster to Polygon § Viewsheds § Watersheds § Raster to Polygon GEOG 380 - Topic 11 § Point to Raster § Interpolation tools: Kriging, IDW, Splines, etc. § Density tools: Kernel density § Point to Raster § Polyline to Raster § Polyline to Raster § Polygon to Raster § Polygon to Raster § Topo to Raster 7 GIS Analysis Terminology § Entity: An individual point, line, or polygon in a GIS database § Attribute: Data about the entity § In a vector GIS, attributes are stored in a database, and there may be many attributes for any given entity § In a raster GIS, the grid code (x, y) represents the spatial attribute for the pixel, and there typically is only one (non-spatial) attribute per pixel i.e., height, brightness, temperature, etc. GEOG 380 - Topic 11 8 … continued § Data layer/coverage: one of a series of themes or data sets in a GIS, consisting of entities and their associated attributes § Note: the general term ‘coverage’, as used here, is actually a generalization of the original term, which refers to the Arc/Info native file format. However, it’s used so often in this context (data layer) that I’m including it. https://vimeo.com/53016715 § Image: a raster data layer § Note that images (rasters) are not limited to remote sensing imagery. Anything that we store ‘pixel by pixel’ is a raster GEOG 380 - Topic 11 9 GIS Analysis Terminology, Cont… § Function/operation: a data analysis procedure performed by a GIS § Algorithm: an unambiguous sequence of functions/operations designed to solve a problem GEOG 380 - Topic 11 10 Queries § Queries: questions posed to the database that do not involve changes to the database. § There are many ways of interrogating the database for information in formats that would qualify as queries § § § § § ‘Right-click: properties’ on individual layers Cursor coordinates ‘Identify’ mode Histograms, scatterplots, and other charts/graphs Formal database queries GEOG 380 - Topic 11 11 … continued § Two basic types: spatial and aspatial § Spatial queries operate “by location” (found under the Selection menu in ArcGIS Pro) § Select features that intersect… § “ ” that are within a distance of… § “ ” that are completely contained by… § “ ” that contain… § Etc… GEOG 380 - Topic 11 12 … continued § Aspatial queries interrogate the attribute tables through structured query language (SQL) § SELECT … FROM … WHERE … § Found under the Selection menu in ArcGIS Pro GEOG 380 - Topic 11 13 Boolean Expressions § § Boolean logic is based on the evaluation of binary values (yes/no, on/off, 1/0) Complex queries can be created by combining individual expressions (spatial or aspatial) through boolean operators: § § § § GEOG 380 - Topic 11 AND: true if both are true OR: true if either is true XOR: true if exactly one is true NOT: inversion expression 14 Measurements § Measurements: simple measurements regarding geographic entities or attributes § The ability of computers to make accurate and consistent area measurements was the original motivations for the Canada Geographic Information System (CGIS): the world’s first GIS https://www.esri.com/arcgis-blog/products/arcgis-earth/3dgis/perform-interactive-measurements-in-arcgis-earth/ § Common GIS metrics: length, perimeter, and area § Also, many others (shape, aggregation, interspersion, etc.), though not as common GEOG 380 - Topic 11 15 Distance Measurements § Simplest method of determining distance between points in space is the Pythagorean or Euclidean distance § Distance between A and B defined by hypotenuse of a right-angle triangle § Assumption is that the points lie on a flat plane, and, of course, the world is not flat! A B GEOG 380 - Topic 11 16 … continued § Great circle distance calculates the metric for a spherical earth § Again, though, the earth is not completely spherical (geoid!), so even more complex procedures may be called for under certain circumstances https://users.cs.jmu.edu/bernstdh/web/common/lectures/su mmary_great-circle-distance_spherical.php GEOG 380 - Topic 11 17 … continued § Manhattan distance calculates the distance along raster cell sides § “The distance between two points measured along axes at right angles.” § This could result in a length distortion created by the raster data model vs a vector model GEOG 380 - Topic 11 18 Distance Measurements, Cont… § Remember, ALL measurements in GIS are an approximation, due to several factors ESRI online help § Vector: lines and polygons are made up of straightline segments (polylines) § Raster: all entities (points, lines, polygons) are represented with pixels (i.e., small squares) § Distortions inherent in the projection, and because of slope, and elevation differences. § Generalization of complex objects by simple models GEOG 380 - Topic 11 19 Area & Perimeter Measurements § GIS perimeter estimates are a fairly straightforward extension of distance measurements § Summing up the lengths of the polylines defining the polygon in a vector layer § Multiplying the number of cell sides that make up the boundary of the feature by the pixel size § Similarly, area measurements are relatively simple extensions of geometric logic https://rdkb.sgrc.selkirk.ca/Help/Content/Client_APIs/SV_User/ SVU_MeasureDistanceOnTheMap.htm GEOG 380 - Topic 11 20 Transformations § Transformations: simple methods of spatial analysis that transform attributes or entities of geographic data into useful NEW products https://desktop.arcgis.com/en/arcmap/latest/manage-data/using-sqlwith-gdbs/spatial-operation-functions-for-st-geometry.htm GEOG 380 - Topic 11 § Buffers, overlays, point-in-polygon, interpolation: these relatively simple operations form the basis of powerful algorithms that solve a multitude of geographic problems 21 Transformations: Buffering https://saylordotorg.github.io/text_essentials-of-geographic-information-systems/s11-01-single-layer-analysis.html § A buffer operation builds NEW objects around existing point, line, or polygons by identifying all areas within a specified distance § Among the most useful and popular GIS operations § Identifying danger zones in an industrial accident § Estimating customers within driving distance of a retail store § Avoiding protected riparian areas for logging operations GEOG 380 - Topic 11 22 … continued § Buffering can be done with both raster and vector layers, but rasters enable the simple extension of buffering to consider factors other than simple distance § E.g., buffering access to a city in a manner that incorporates travel speed, by assigning a friction cost to each cell http://www.geog.ucsb.edu/~good/176b.old/10.html#1 GEOG 380 - Topic 11 23 Transformations: Point in Polygon § The point-in-polygon operation determines whether a point lies inside or outside a polygon, though it can be extended to consider many points and many polygons § Points could represent instances of disease in a population; polygons health districts. Which districts are affected by a particular outbreak? § Points could represent rare animal sighting locations; polygons represent management districts. What areas should be subject to special species-at-risk regulations? GEOG 380 - Topic 11 24 Transformations: Point in Polygon § Standard algorithm for determining whether a point lies within a polygon involves extending a line from the point towards infinity § If the line crosses the polygon’s boundary/edge an odd number of times, or if it lies on the polygon edge: it is in § If it crosses an even number of times (or zero): it’s out http://www.geog.ucsb.edu/~good/176b.old/10.html#1 GEOG 380 - Topic 11 25 Transformations: Polygon Overlay § Polygon overlays involve the spatial intersection of two different polygon layers, and are among the most varied and powerful families of GIS operations http://www.geog.ucsb.edu/~good/176b.old/10.html#1. § What proportion of a protected area was affected by a forest fire? § What is the total area of forest owned by each property holder? GEOG 380 - Topic 11 26 ESRI online help Overlay operations GEOG 380 - Topic 11 27 Polygon Overlay Issues § § Polygon overlay operations with vector datasets are problematic because of the potential for slivers being generated by the procedure The same boundary (e.g., a coastline) may be represented slightly different in each layer, because of slight digitizing errors § Paradoxically, the more detail you put into the digitization, the more pronounced this problem becomes! https://www.onestopgis.com/GIS-Theory-and-Techniques/Spatial-Data-Editing/TopologicalErrors/2-Types-of-Topological-Errors-with-Geometric-Features.html § Most software packages try to minimize this problem though tolerance thresholds, but these must be set correctly Ø Not an issue in raster overlays, since the entities (pixels) are all the same GEOG 380 - Topic 11 28 Transformations: Spatial Interpolation § Spatial interpolation: algorithms that attempt to estimate values of a continuous field in places where no measurements exist in the database § Estimating rainfall in places away from individual weather stations § Estimating elevation from contour lines https://gisgeography.com/kriging-interpolation-prediction/ GEOG 380 - Topic 11 29 https://www.angelchen.net/blog/2018/1/11/spatially-interpolating-snow-water-equivalent-data-in-the-canadian-prairies Transformations: Spatial Interpolation § Spatial interpolation: algorithms that attempt to estimate values of a continuous field in places where no measurements exist in the database § Estimating rainfall in places away from individual weather stations § Estimating elevation from contour lines § GEOG 380 - Topic 11 Thiessen polygons, inverse-distance weighting, kriging: topics for another day! 30 GIS Analysis: Summary § Queries, measurement, and transformations: simple GIS analysis functions that, if clearly understood, can be strung together into powerful algorithms capable of solving sophisticated geographic problems § Much more to learn on this subject: stay tuned for GEOG 482! GEOG 380 - Topic 11 31