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GIS > Midterm > Flashcards

Midterm Flashcards

1
Q

What is geographic information science?

A
  • 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
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2
Q

What is a geographic information system?

A

A computer-based system to aid in the collection, maintenance, storage, analysis, output, and distribution of spatial data and information

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3
Q

What ‘level’ is spatial representation on?

A

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

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4
Q

Discuss the conceptual representations of geographic phenomenon.

A

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)

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5
Q

What is the false dichotomy between the object and field views?

A

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)

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6
Q

What ‘level’ does spatial data models fall on?

A

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)

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7
Q

What are the spatial data models?

A

Vector and Raster.

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8
Q

How does each spatial data model represent space?

A

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)

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9
Q

Which view does each spatial data model represent best?

A

Vector: object view - best for discrete data with well-defined boundaries
Raster: field view - best for continuous data

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10
Q

Are spatial data georeferenced to a geographic of a projected coordinate system?

A

Yes

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11
Q

Are spatial data defined by a particular map scale (original scale of capture)?

A

Yes

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12
Q

Are spatial data smaller or larger than the reality they represent and how does this affect the amount of detail?

A

They are smaller than the reality they represent and this limits the amount of detail.

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13
Q

Do spatial data include every piece of information?

A

There is some level of generalization or simplification - error of omission.

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14
Q

How is spatial data flawed?

A

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)

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15
Q

What ‘level’ is the spatial data structure on?

A

A physical level

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16
Q

Explain the vector dimensionality and properties and how it is influenced by map scale.

A

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)

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17
Q

What is a spaghetti vector model? (simplest of vector structures)

A

An unstructured method of representing simple points, lines and polygons no spatial relationships (topology) are explicitly defined or encoded with data

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18
Q

What is the common data structure of spaghetti vector models?

A

Shapefiles or CAD files

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19
Q

What is topology and its relationships?

A

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.

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20
Q

What are the three topological relationships? (Three C’s)

A

Connectivity, Contiguity and Containment

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21
Q

What is a topological vector model?

A

A model that uses related tables to explicitly define and encode spatial relationships

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22
Q

How does a topological vector model store information?

A

Directionality! From left and right polygons and from (start) and to (end) nodes.

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23
Q

For a complex vector structure what is the most common approach? For a common?

A

Arc-node model. Common: Arcinfo coverage.

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24
Q

What is an object-based vector model?

A

Based upon an abstraction of features into database objects stored in a relational database management system

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25
Q

How do objects integrate features in an object-based vector model?

A

Through spatial (topological), relationship classes and attribute domains - all stored in tables

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26
Q

What is a common data structure for an object-based vector model?

A

Geodatabase feature class and feature dataset

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27
Q

What are vector attributes?

A

Attributes that share a one-to-one relationship between spatial features and their corresponding record in the value attribute table.

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28
Q

Why is it significant that attributes share a one-to-one relationship?

A

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

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29
Q

What is TIN and what does it model?

A

Triangulated irregular network and it models surfaces - continuous data such as elevation.

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30
Q

What is TIN based on?

A

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.

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31
Q

How is raster data represented?

A

Cells - each cell can have only one value stored as a number

32
Q

What is raster resolution and what is it a function of? Can it be increased?

A

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.

33
Q

What is the mixed pixel problem associated with cell value assignment?

A

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.

34
Q

How are raster attributes stored?

A

They are stored in cells as numbers - either integer for discrete rasters or floating point for continuous, statistical surfaces

35
Q

Does each raster cell have to have an attribute value?

A

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.

36
Q

What sort of a relationship between cells and their corresponding value in the value attribute table do these data have?

A

A many to one relationship.

37
Q

Why is it significant that vector data has a many to one relationship?

A

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.

38
Q

List the relevant components of a spatial referencing system.

A

Ellipsoid -> datum -> geographic coordinate system -> projected coordinate system

39
Q

What is an ellipsoid?

A

A mathematical estimation of earths general shape - smooth

40
Q

What does an ellipsoid establish?

A

The reference system for measuring horizontal location.

41
Q

Why do we have so many different ellipsoids?

A

DUNNO Differences in different places or what

42
Q

What does a horizontal datum do?

A

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

43
Q

How does a horizontal datum work?

A

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

44
Q

What are the two types of centered datums did we discuss?

A

Local and Geocentric

45
Q

What is a local datum? Give an example of one.

A

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.

46
Q

What is a geocentric datum? Give an example of one.

A

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.

47
Q

What is a datum shift?

A

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.

48
Q

What is a geographic (datum) transformation?

A

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.

49
Q

What is a geographic coordinate system?

A

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

50
Q

Is a geographic coordinate system the same as a cartesian coordinate system?

A

No

51
Q

How does a map projection and projected coordinate systems work?

A

The transfer the geographic coordinate system established by a datum to a flat surface - 3D -> 2D

52
Q

What are four spatial properties that are inevitably distorted on a map?

A

Shape, Area, Distance and Direction

53
Q

What are the projections by surface?

A

Conical, Cylindrical and Azimuthal or Planar.

54
Q

What are the projections by preservation of property?

A

Equal area (preserves area), conformal (preserves local shape), equidistant (preserves distance), true-direction or azimuthal (preserve direction)

55
Q

How/why are lines tangency significant to map projections?

A

The lines are points where the map has no distortion or are areas of true scale - distortion increases away from the lines of tangency.

56
Q

How can lines of tangency intersect?

A

In one place - Tangent; in two places - Secant

57
Q

What is a linear unit of project?

A

For x and y what are the units being measured in - feet or meters?

58
Q

What is a geoid?

A

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)

59
Q

What is a database?

A

A structured set of data usually organized into tables; a matrix of columns and rows

60
Q

What is made up of?

A

Entity - theme/table; Instance or row - record; column - field

61
Q

What is a DBMS?

A

An application designed to organize the storage, retrieval and modification of data.

62
Q

What is a relational database management system?

A

Based on a database design technique called normalization which removes repeating columns and repeating rows (duplicated data) and relates the resulting tables using keys .

63
Q

What are keys?

A

They are unique ID’s or fields with common info.

64
Q

Define primary key.

A

A field that uniquely identifies each record in a primary (destination) table

65
Q

Define a foreign key.

A

Is field from second (source) table used to link it to a primary (destination) table through its primary key

66
Q

What are spatial databases?

A

Collection of data that include both spatial and attribute stored in related database tables or files

67
Q

How do raster and vector store info in spatial databases?

A

Vector data is stored in feature attribute table (FAT), raster store attributes in a value attribute table (VAT)

68
Q

Why is there no VAT for continuous raster data?

A

Because there is an infinite number of values

69
Q

What is a join and relate?

A

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.

70
Q

Define a join.

A

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.

71
Q

When is a join usually used?

A

To attach attributes to the attribute table of a geographic layer by appending the fields of one to the other.

72
Q

Define a relate.

A

one to many & many to many. one (or many) records in the destination table has many records related to it from the source table.

73
Q

When is relate used?

A

When you want to not append the layers attribute tables but want to access the related data when necessary

74
Q

What is a spatial join?

A

This appends the attributes from one layers attribute table to another layers attribute table based on the relative spatial locations of the two layers.

75
Q

What are the appends based on spatial relationships?

A

Intersect -> features that share a common part with others
Containment -> feature that contain or surround others
Proximity -> features within a distance of

76
Q

What are the appends based on cardinality of the relationships?

A

Polygons to points: many-to-one, a simple join

points to polygons: one-to-many, a summary join

GIS (3 decks)

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