Data Models

GIS I

Data Models

• GIS is an abstraction of reality.• A perfect copy of reality cannot be recreated in

the computer.• We create models – sets of constructs for

describing & representing select aspects of the real world in a computer.

• Models are composed of a mix of raster, vector, and attribute data.

• Model is tailored to a specific function.

Coding Vector Data

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Reality Vector Mode Model of Reality

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A topologic vector model records the points and linesshared between polygons as unique items, thus every oneof the points and lines are recorded in the data only once.

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Polygon 2 is on the right sideof the line ABCED.

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Polygon 2 knows it’s adjacent to Polygons 1, 3, & 4.It shares a line segment with each.

Polygon 2 knows it touches Polygon 5.It shares node E with Polygon 5.

What is Topology?

• Shared Geometries, Adjacency and Overlap• Where points, lines, and polygons share

individual vertices. Move a point and it moves a vertex in a line/polygon, and vice versa.

• Two polygons that share vertices are considered adjacent.

• Overlapping (or non-overlapping) features can be located, and then marked as errors.

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Strict Topology

• Features are composed from a common set of points and lines.

• Altering the vertices of one polygon affects polygons that share those vertices.

• Harder to introduce gaps or slivers.

Topology

• Can you think of a reason why topology would be important to model?

• Where in the real world is this concept important?

• Let’s take a look at some examples.

Cadastre Example

benchmark survey (COGO) parcels zones

Parcel Overlap Example

• The boundaries of two properties should never overlap, and there should never be a gap between them, unless intentional.

• Clear error in parcel boundaries.

Policy-based Topology Rules

• In the NJ State Plan, CESs and the Environmentally Sensitive Planning Area both represent areas of environmental importance.– Thus, CESs should never be placed on top of the ES

Planning Area.• In our utility network, poles hold up the

transmission lines.– The transmission line features must always share a

vertex with the utility pole point features.

GIS is extensible

• With modern GIS, a polygon is not just a polygon.• Software can be adapted to fit your model of

reality.• The software can be easily extended to create

new data types and perform new analyses. • GIS can be adapted to store, model, and display

data about any observable phenomenon on the Earth.

Objects

• GIS Features as Objects is a recent method of representing aspects of the real-world in GIS

• Example of the shift from specialty data to DBMS that are spatially-aware

• Non-planar, temporally shifting, topologically linked, rule-based actions

• Still important to check for topology to ensure as a quality control step

Vector Geometry as Objects

• Parcels– Planar geometries with attribute information

• Parcels as objects in a Cadastral “carpet”– Objects with topology rules (“don’t overlap, unless”)– Members of “regional” features (zoning, municipality)– Composed of surveyed parts (COGO, benchmarks)– Keys that link to attribute tables (owner(s),

assessments, plans, etc)

Attributes as Objects

• Not only can multiple sets of geospatial features interact with rules, the attributes can be linked with one another, with their own set of rules and actions

• Ownership record linked to GIS parcel– Search on multiple owners, records– Removal of parcel warns about “orphan” owner

• Functions that can be performed by GIS analyst can be embedded in the actual database

Explore Models

• Let’s take a look at several GIS data models.• Take note of the storage method:– Raster– Vector (and vector type: point, line, polygon, etc…)

• Also take note of the model family:– Topological Model– Object Model– Both

Elevation using LIDAR

• LIDAR data is 3D elevation data recorded from an airplane.

• Stored as “mass points” – even a small area is composed of thousands of point features.

• No real need for attributes, simply XYZ points.

• Points can be joined together to create a surface model of a landscape.

Elevation DEMs

• Digital Elevation Models, or DEMs, often refer to a raster representation of elevation.

• Each cell in the raster grid contains a value that is the height of the cell above a fixed point (i.e. sea level).

Elevation using TINs

• Triangulated Irregular Networks, or TINs are vector models that represent elevation.

• The study area is composed of individual triangles, composed of a network of shared nodes and edges

• The surfaces of the triangles attempt to represent the surface, so in areas of gradual elevation change, there are fewer triangles.

TIN Model of Campus

TIN Model of Campus

Networks

• Analysis can be performed across a network, represented by a feature dataset of points and lines.

• Road network or water, sewer, utility, rail, etc…• Optimal route – shortest, lowest cost, avoiding

left turns, follow height and weight restrictions, time of day restrictions, include real-time traffic…

• Multi-modal – walk/bike to bus stop, bus to train, walk from train to final destination.

Models Diagrammed

• GIS models can be depicted in a schematic form, similar to a flow chart.

• Shows the interconnected nature of the classes that make up the overall model.

• Some models can be constructed within ArcGIS using ModelBuilder.

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Creating GIS Models

• Abstractions of reality naturally have shortcomings.

• Models tailored to a specific task can be used to explore phenomenon or predict effects.

• Developing a data model to solve a problem is how GIS has become a decision-making platform.

• Consider how you could study an abstract set of data using GIS to solve real-world issues.