Midterm Flashcards
What is geographic information science?
- A fundamental field of study which examines the representation, storage, analysis and visualization of
geographic information - At is a basic research field that seeks to redefine geographic information/concepts and its use in the
context of GIS
What is a geographic information system?
A computer-based system to aid in the collection, maintenance, storage, analysis, output, and distribution of spatial data and information
What ‘level’ is spatial representation on?
The conceptual level - real-world things are entities (geographic themes); conceptualizations of spatial entities governed by how we perceive or interpret that entity and its intended application in GIS
Discuss the conceptual representations of geographic phenomenon.
Object and field are the conceptual representations. Object is made up of discrete entities with boundaries and location explicitly defined (modeling the human/urban landscape dominates this view). Field is a collection of spatial distributions that vary continuously across space (modeling the natural environment dominates this view)
What is the false dichotomy between the object and field views?
Both views can be used to represent the same entity, they are not mutually exclusive in representing geographic phenomenon - depends on users perception of the phenomenon (a lake can be defined by boundaries i.e. discrete or its distribution on a lakeness scale i.e. continuous)
What ‘level’ does spatial data models fall on?
The logical level, it is a representative form of the conceptual view. Real world observations are modeled by spatial geometry (coordinates/georeferenced) and attributes (data and text describing the characteristics of the features)
What are the spatial data models?
Vector and Raster.
How does each spatial data model represent space?
Vector: points, lines polygons; locations are explicitly defined by pairs of coordinates. (point is basic unit)
Raster: cells; location implicit by the size/area of the cell and the cell layout (cell is basic building block)
Which view does each spatial data model represent best?
Vector: object view - best for discrete data with well-defined boundaries
Raster: field view - best for continuous data
Are spatial data georeferenced to a geographic of a projected coordinate system?
Yes
Are spatial data defined by a particular map scale (original scale of capture)?
Yes
Are spatial data smaller or larger than the reality they represent and how does this affect the amount of detail?
They are smaller than the reality they represent and this limits the amount of detail.
Do spatial data include every piece of information?
There is some level of generalization or simplification - error of omission.
How is spatial data flawed?
Because of our conceptualization of an entity, our definition of an entity, its intended purpose in GIS, and/or by the limits in technology for capturing that entity as data (i.e. satellite resolution)
What ‘level’ is the spatial data structure on?
A physical level
Explain the vector dimensionality and properties and how it is influenced by map scale.
Zero dimensional: no length or area - just a point or a node
One dimensional: length, no area - line segment or an arc
Two dimensional: length and area - polygon
The dimensionality is influenced by scale - 1:5,000,000 river is a point, 1:500,000 river is a line & 1:5,000 river is a polygon)
What is a spaghetti vector model? (simplest of vector structures)
An unstructured method of representing simple points, lines and polygons no spatial relationships (topology) are explicitly defined or encoded with data
What is the common data structure of spaghetti vector models?
Shapefiles or CAD files
What is topology and its relationships?
The spatial relationships between features that do not change - a set of rules and behaviors that model how points, lines, and polygons share geometry.
Relationships: Connectivity, contiguity and containment.
What are the three topological relationships? (Three C’s)
Connectivity, Contiguity and Containment
What is a topological vector model?
A model that uses related tables to explicitly define and encode spatial relationships
How does a topological vector model store information?
Directionality! From left and right polygons and from (start) and to (end) nodes.
For a complex vector structure what is the most common approach? For a common?
Arc-node model. Common: Arcinfo coverage.
What is an object-based vector model?
Based upon an abstraction of features into database objects stored in a relational database management system
How do objects integrate features in an object-based vector model?
Through spatial (topological), relationship classes and attribute domains - all stored in tables
What is a common data structure for an object-based vector model?
Geodatabase feature class and feature dataset
What are vector attributes?
Attributes that share a one-to-one relationship between spatial features and their corresponding record in the value attribute table.
Why is it significant that attributes share a one-to-one relationship?
There is always one record in the attribute table that corresponds to a feature on the map allowing for multiple records in the table to have the same ‘unique’ value. If a record is deleted so is its corresponding counterpart
What is TIN and what does it model?
Triangulated irregular network and it models surfaces - continuous data such as elevation.
What is TIN based on?
Irregularly spaced points with associated coordinates in three dimensions (x, y, and z) connected by edges (lines) that form a network of non-overlapping triangles.
How is raster data represented?
Cells - each cell can have only one value stored as a number
What is raster resolution and what is it a function of? Can it be increased?
The amount of detail the raster dataset contains. Is a function of the cell size/area at the original state of capture. Not by simply “resampling” to a smaller cell size.
What is the mixed pixel problem associated with cell value assignment?
Cells are characterized by size and occupy area, thus assigning a single attribute value to a cell covering an area with heterogeneous (multiple) classes, i.e. water-ag-forest, in the real world is problematic.
How are raster attributes stored?
They are stored in cells as numbers - either integer for discrete rasters or floating point for continuous, statistical surfaces
Does each raster cell have to have an attribute value?
Yes, if the data is missing or null than it will be defined as NoData value i.e. -9999 or some other unlikely number to be used in the true dataset.
What sort of a relationship between cells and their corresponding value in the value attribute table do these data have?
A many to one relationship.
Why is it significant that vector data has a many to one relationship?
The table has one record for each unique value in the grid, so when making a selection with raster data you are selecting only on record in the table.
List the relevant components of a spatial referencing system.
Ellipsoid -> datum -> geographic coordinate system -> projected coordinate system
What is an ellipsoid?
A mathematical estimation of earths general shape - smooth
What does an ellipsoid establish?
The reference system for measuring horizontal location.
Why do we have so many different ellipsoids?
DUNNO Differences in different places or what
What does a horizontal datum do?
Ties an estimated ellipsoid to the earth by ‘fixing’ it to the earths surface through a physical network of precisely measured points -> Sets the position and orientation of an ellipsoid relative to the earths center
How does a horizontal datum work?
It defines the origin of where graticules of latitude and longitude will lie on the earths surface, i.e. the origin for a geographic coordinate system
What are the two types of centered datums did we discuss?
Local and Geocentric
What is a local datum? Give an example of one.
Based on astronomical observations, local surface measurements and older estimated ellipsoids - only able to fit in one particular portion of the earth - the origin (center) is usually offset relative to the earths true center. North American Datum 1927.
What is a geocentric datum? Give an example of one.
Based on satellite measurements and newer ellipsoid estimates - aligned to fit the earths surface globally and as a result the origin (center) is aligned to earths true center of mass. North American Datum 1983 and WGS 1984.
What is a datum shift?
This is a change in the estimation of where the feature is truly located - so the actual thing hasn’t moved just its estimated spot in space.
What is a geographic (datum) transformation?
Coordinates are based off datums and each datum is based on one particular ellipsoid, a change in a datum changes the underlying ellipsoid and thus the origin of the spatial reference system based off of it. - > transformation changes latitude and longitude form one datum to another using math, yo.
What is a geographic coordinate system?
A spherical system based on angles from an origin. Latitude and longitude are measured as degrees or angles from the center of the earth - a geographic coordinate system is always established by a datum
Is a geographic coordinate system the same as a cartesian coordinate system?
No
How does a map projection and projected coordinate systems work?
The transfer the geographic coordinate system established by a datum to a flat surface - 3D -> 2D
What are four spatial properties that are inevitably distorted on a map?
Shape, Area, Distance and Direction
What are the projections by surface?
Conical, Cylindrical and Azimuthal or Planar.
What are the projections by preservation of property?
Equal area (preserves area), conformal (preserves local shape), equidistant (preserves distance), true-direction or azimuthal (preserve direction)
How/why are lines tangency significant to map projections?
The lines are points where the map has no distortion or are areas of true scale - distortion increases away from the lines of tangency.
How can lines of tangency intersect?
In one place - Tangent; in two places - Secant
What is a linear unit of project?
For x and y what are the units being measured in - feet or meters?
What is a geoid?
Sea surface is a function of gravity - no tidal, atmospheric, or surface influence.
The surface is irregular, establishes the reference system for measuring vertical location (elevation)
What is a database?
A structured set of data usually organized into tables; a matrix of columns and rows
What is made up of?
Entity - theme/table; Instance or row - record; column - field
What is a DBMS?
An application designed to organize the storage, retrieval and modification of data.
What is a relational database management system?
Based on a database design technique called normalization which removes repeating columns and repeating rows (duplicated data) and relates the resulting tables using keys .
What are keys?
They are unique ID’s or fields with common info.
Define primary key.
A field that uniquely identifies each record in a primary (destination) table
Define a foreign key.
Is field from second (source) table used to link it to a primary (destination) table through its primary key
What are spatial databases?
Collection of data that include both spatial and attribute stored in related database tables or files
How do raster and vector store info in spatial databases?
Vector data is stored in feature attribute table (FAT), raster store attributes in a value attribute table (VAT)
Why is there no VAT for continuous raster data?
Because there is an infinite number of values
What is a join and relate?
Establishing a temporary link between records in two separate tables through an attribute (key) common to both. Virtually appends attributes from one table to another.
Define a join.
One-to-one or a many-to-one. many (or one) records in the destination table have exactly one record going to it from the source table.
When is a join usually used?
To attach attributes to the attribute table of a geographic layer by appending the fields of one to the other.
Define a relate.
one to many & many to many. one (or many) records in the destination table has many records related to it from the source table.
When is relate used?
When you want to not append the layers attribute tables but want to access the related data when necessary
What is a spatial join?
This appends the attributes from one layers attribute table to another layers attribute table based on the relative spatial locations of the two layers.
What are the appends based on spatial relationships?
Intersect -> features that share a common part with others
Containment -> feature that contain or surround others
Proximity -> features within a distance of
What are the appends based on cardinality of the relationships?
Polygons to points: many-to-one, a simple join
points to polygons: one-to-many, a summary join
