Topic 10: Vector Analysis Flashcards
Recognize some of the options and strategies for three major categories of GIS analysis operations: queries, measurement, and transformations
Queries: asking a question and having the dataset answer, does not change the data set
- spatial: based on a location
- spatial: interrogate the attribute tables through query language
Measurements: determining distance
- Pythagore method, flat surface
- great-circle distance, although earth is not a perfect circle
- manhattan calculates distances using the edge of raster cells
transformations
- buffers and overlay and point-in-polygon
Perform some simple overlay and buffering operations
overlay:
- intersect: computes the intersection of the two layers
- clip: takes out point of interest
- erase: erases where the feature layer is placed
- union: combining the two layers
- identity: two layers intersect, input layer will get the identities of the identity feature where they overlap
buffer:
- builds object around existing point, line, or polygon
- can be done with both raster and vector
Processing in Raster & Vector
- Beginning 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?
Actually, most operations can be accomplished with both data models
Conversion between the two can introduce errors
Often best to stick with the “native” data format
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 given entity
- In a raster GIS, the grid code represents the attribute for the pixel, and there typically is only one attribute per entity
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 I’ve used here, is actually a bastardization 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
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
function/ operation
a data analysis procedure performed by a GIS
Algorithm
an unambiguous sequence of functions/ operations designed to solve a problem
Queries
Questions posed to the database that do not involve changes to the database
What is the difference between spatial and aspatial?
Spatial queries operate “by location” (found under the selection menu in ArcGIS Pro)
Aspatial queries interrogate the attribute tables through structured query language (SQL)
What are the different Boolean expressions?
Boolean logic is based on the evaluation of binary values (yes/no, on/off, 1/10)
Complex queries can be created by combining individual expressions (spatial or aspatial) through boolean operators
- AND: true if both are true
- OR: true if either is true
- XOR: true if exactly one is true
- NOT: inversion expression
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
How to determine distance measurements using the simplest method?
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!
What is the great circle distance?
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
What is Manhattan distance?
Manhattan distance calculates the distance along raster cell sides
Again could result in a distortion created by the data model
Area and 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
Transformations
simple methods of spatial analysis that transform attributes or entities of geographic data into useful NEW products
Buffers, overlays, point-in-polygon, interpolation: these relatively simple operations form the basis of powerful algorithms that solve a multitude of geographic problems
Buffers
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
Buffers: Raster vs Vector
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 of buffering to consider factors other than simple distance
the application on a raster is more flexible in some applications
Point in polygon
the point-in-polygon operation determines wheter a point lies inside or outside a polygon, though it can be extended to consider many points and many polygons
How does the algorithm determine if a point is in a polygon?
There is a line that extends for infinity, if it crosses the polygon an odd number of times it is in the polygon. If it crosses an even amount of times it is outside the polygon
Polygon overlay
involve the spatial intersection of two different polygon layers, and are among the most varied and powerful families of GIS operations
overlay operations: identity
- Take cookie cutter, plop it over top, creating breaks in the underlying data
- Make a break saying that segment contains attribute of cookie cutter
- Adds attribute
overlay operations: intersect
- computes spatial intersection of the two layers
- where they are the same (Boolean logic)
- clips and combines
overlay operations: union
both of them combined into one feature layer
overlay operations: clip
taking out area of interest
overlay operations: erase
(opposite of clip) erasees where the feature layer is placed
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 (eg., coastline) may be represented slightly different in each layer, because of the slight digitizing errors
- Paradoxically, the more detail you put into the digitization, the more pronounced this problem becomes
Most software packages try to minimize this problem though tolerance thresholds, but these must be set correctly
- Not an issue in raster overlays, since the entities (pixels) are all the same
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
Inverse distance weighted function
- measure straight line distance between points, distance weighted average
- one of the most common
