GEOG 380 - Topic 09 - Vector analysis

Vector Analysis

What are the three major categories of GIS analysis covered in this lecture?

Queries, measurements, and transformations.

What is a query in GIS?

A question asked of the GIS database that retrieves information without changing the database.

What is the difference between a spatial query and an aspatial query?

A spatial query works by location or spatial relationship, while an aspatial query works on attribute table information, often using SQL.

What is an entity in GIS?

An individual point, line, or polygon in a GIS database.

What is an attribute in GIS?

Information stored about a geographic entity. In vector GIS there can be many attributes per feature; in raster GIS the grid code usually stores the main attribute

What is a data layer in GIS?

One theme or dataset in a GIS, made up of entities and their associated attributes.

What is the difference between raster and vector data in analysis?

Vector represents discrete features as points, lines, and polygons, while raster represents space as pixels or grid cells. Both can do many of the same analyses, but conversion between them can introduce errors.

Why is it often best to keep data in its native format?

Because converting between raster and vector can introduce errors.

What is a GIS function or operation?

A data analysis procedure performed by a GIS.

What is an algorithm in GIS?

An unambiguous sequence of GIS operations designed to solve a problem.

What is Boolean logic used for in GIS?

It is used to combine query conditions into more complex expressions.

What does the Boolean operator AND mean?

Both conditions must be true

What does the Boolean operator OR mean?

At least one condition must be true

What does the Boolean operator NOT mean?

It reverses or excludes a condition.

What kinds of measurements are common in GIS?

Length, perimeter, and area.

What is Euclidean distance?

Straight-line distance on a flat plane, based on the Pythagorean theorem.

When is Euclidean distance most appropriate in GIS?

When working on a projected flat surface over a relatively small area where Earth curvature is negligible. Inference supported by the lecture’s flat-plane description.

What is great circle distance?

The shortest distance between two points on a spherical Earth.

Why is great circle distance still an approximation?

Because Earth is not a perfect sphere; it is more accurately described as a geoid.

What is Manhattan distance?

Distance measured along raster cell sides, like moving in a grid.

Why are all GIS measurements approximations?

Because vector features are simplified into straight segments, raster features into pixels, and measurements are affected by projection distortion and slope.

How is polygon perimeter calculated in vector GIS?

By summing the lengths of the polylines that define the polygon boundary.

How is polygon perimeter estimated in raster GIS?

By multiplying the number of cell sides along the boundary by the pixel size.

What is a transformation in GIS?

A spatial analysis method that transforms attributes or entities into useful new products.

What are some examples of GIS transformations from the lecture?

Buffers, overlays, point-in-polygon, and interpolation.

What is a buffer in GIS?

A new object or zone created around point, line, or polygon features within a specified distance

Give one real-world use of buffering.

Identifying danger zones around industrial accidents, customers within driving distance of a store, or protected riparian areas near streams.

What advantage do rasters have for buffering?

Raster buffering can be extended to include friction or travel cost, not just simple straight-line distance.

What is point-in-polygon analysis?

A GIS operation that determines whether a point lies inside or outside a polygon.

How does the standard point-in-polygon algorithm work?

A line is extended from the point toward infinity. If it crosses the polygon boundary an odd number of times, the point is inside; if even or zero, it is outside.

What is polygon overlay?

The spatial intersection and combination of two polygon layers to produce a new layer with combined geometry and attributes.

What does Intersect do in overlay analysis?

It keeps only the portions shared by both input layers.

What does Union do in overlay analysis?

It keeps the input features and adds attributes from the identity layer where overlap occurs

What does Clip do in overlay analysis?

It cuts an input layer down to the boundary of another layer

What does Erase do in overlay analysis?

It removes the areas of the input layer that overlap with the erase layer.

What is a sliver polygon?

A small unwanted polygon created during vector overlay because boundaries in different layers do not line up perfectly.

Why do slivers happen in vector overlay?

Because the same boundary may be digitized slightly differently in each layer due to small errors or differences in detail

Why is sliver creation less of an issue in raster overlays?

Because raster cells all align on the same grid.

What is spatial interpolation?

A set of algorithms used to estimate values of a continuous field in places where no measurements exist.

Give two examples of interpolation problems.

Estimating rainfall between weather stations and estimating elevation from contour lines.

What interpolation methods are named in the lecture?

Thiessen polygons, inverse-distance weighting, and kriging.

Why is interpolation used for continuous data rather than discrete features?

Because it estimates values across a continuous surface between known sample points.

What is the main takeaway from the lecture summary?

Simple GIS functions like queries, measurements, and transformations can be combined into powerful algorithms for solving complex geographic problems.