Introduction to GIS

The two common GIS data models
Vector and Raster.

Data model
defines how to abstract real-world features into a format that can be understood by a computer.

Vector Data Model
The world is represented as objects with  well-defined boundaries in empty space

Uses discrete elements to represent  real-world entities - points, lines and polygons

Organizing Vector Data
Every feature is assigned a unique identifier, which is stored with the feature record in an attribute table

There are several data models used for vector data such as spaghetti and topological

Data formats are based on each data model such as shapefile for spaghetti and geodatabase, coverage for topological data models

Feature Class
- Basic Storage unit and a collection of features that share the same geometry type, attributes, and spatial reference.
- Some attributes in the feature class tables are automatically created and maintained by the geodatabase:  A. For line feature classes, the geodatabase automatically calculates the length of  each feature and stores that data in a field called Shape_Length. |  B. For polygon feature classes, the geodatabase calculates the perimeter and area of each feature and stores those values in fields called Shape_Length and  Shape_Area, respectively.
- Feature classes can also store annotation. 
- Feature classes may exist independently in a geodatabase as standalone feature classes or they can be grouped into feature datasets.

Spaghetti Vector Data model
- Shapefiles are a common file-based data format used in GIS software.
- A file of spatial data that is a just a collection of coordinate strings
- Each entity (or piece of spaghetti) is represented by one data entry
- There is no topology
- Advantages: simple and efficient for data display
- Disadvantages: inefficient for most types of spatial analysis
- Shapefiles are an implementation of the spaghetti vector model

Shapefiles
Designed by ESRI for display purposes. An individual layer stores a single type of geometry (point, line and polygon). NO topology. Draws relatively fast and 'open' file format.

Topological Vector Data Models
- Topology is the study of geometric properties that do not change when the forms are bent, stretched or undergo similar transformations 
- Invariant properties: Polygon adjacency and Connectivity
- Topology has at least two main advantages: Ensures data quality and Can enhance GIS analysis
- Computational cost
- Significant human effort
- Geodatabases and coverages implement topological vector data models

Advantages and Disadvantages of Vector
- Vectors can represent point, line, and area features very accurately 
- Relationships are computed and stored
- Far more efficient than grids
- Works well with plotting devices and tablet digitizers
- Complicated data structure

Geodatabase
- The geodatabase is the primary data storage format for ArcGIS® software.
- In technical terms, the geodatabase is a relational database, a type of  database composed of various tables that organize data and are linked to  one another. 
- Topology is supported
- Geodatabases are created, managed and edited using the standard menus  and tools in ArcCatalogâ„¢ and ArcMapâ„¢.

Advantages of the Geodatabase
- Your data is stored in one central location instead of being spread out in  different file
- Supports versioning, multi-user edits, client-server architecture, and other mainstream database functionality
- Promotes faster and more accurate data entry and editing than other formats.
- Automatically calculates and maintains geometric values for line and polygon  feature classes—length, perimeter, and area.

What are the different types of geodatabases?
File, Personal and Multiuser.

File Geodatabase
- Can handle large datasets with very fast performance.
- Supports one editor and many readers
- Versioning is not supported

Personal Geodatabase
- It uses the Microsoft Access data format (.mdb file)
- Has a 2-gigabyte (GB) size limit.
- Versioning is not supported

Multiuser ArcSDE Geodatabase
- Are stored in a relational database management system such as DB2,  Oracle, or SQL Server and
- Require ArcSDE® technology, which is included with ArcGIS® Server software.
- Supports versioning and there is no limit to size or numbers of users

Feature Datasets
- A feature dataset stores feature classes that have the same coordinate system.
Feature datasets can store feature classes with different geometry types.
- Feature classes grouped into a feature dataset normally have some kind of spatial relationship to one another.
- Feature datasets are primarily used to store feature classes that have topological relationships.
Examples:
- Streams in a particular watershed are connected to rivers; therefore, streams and rivers are topologically related
- Water mains and water laterals in a city form a geometric network to represent the water system

The raster Data Model
- The earth is represented as a grid  of equally sized cells.
- An individual cell represents a portion of the earth such as a square meter or a square mile.
- Only one x,y coordinate pair is normally present, called the origin, which is use to define the location of every cell.
- Every cell holds a number or an index value standing for an attribute
- A cell has a resolution, given as the cell size in ground units 
- All detail about variation within cells is lost, and instead the cell is given a single value

Grids
-Discrete: Each category has the same cell value (Integer - whole number) and Have an attribute table (in INFO format) ->discrete features such as buildings, roads, and parcels.

- Continuous: Cell values are floating points (numbers with decimals) and Don't have an attribute table - continuous phenomena such as elevation, temperature, and precipitation.

Imagery
- Imagery is a collective term for rasters whose cells, or pixels, store brightness  values of reflected visible light or other types of electromagnetic radiation, such  as emitted heat (infrared) or ultraviolet (UV).
- Aerial photos, satellite images, and scanned paper maps are examples of images  commonly used in a GIS.
- To be displayed as a layer and overlay properly with other map layers, imagery  must be referenced to real-world by assigning a coordinate system.
- ArcGIS supports many imagery formats: tif (Tagged Image File Format),  .sid (LizardTech MrSID), and .jpg (Joint Photographic Experts Group).

Advantages and Disadvantages of Raster
- Rasters are faster
- Maps directly onto computer memory structure (array)
- Natural for scanned or remotely sensed data
- Continuous surfaces
- Easy to understand, read, write, draw
- Spatial analytical operations are faster
- Compression is easier
- Spatial inaccuracies

What are the two different types of Data capture?
Primary Data (Direct Measurement) and Secondary Data (Derivation from other sources).

What is Data transfer?
Importing digital data from other sources.

Primary vector data
Ground Surveying:
- Best way to obtain highly accurate point locations
- Time consuming and expensive
- Theodolites to measure angles and tapes for distances replaced by electro-optical devices (total stations) for both
- Typically used for capturing buildings, property boundaries, manholes, for georeferencing of detailed fine-scale aerial photographs and satellite images, and control points

GPS - Global positioning System

LiDAR - Light Detection and Ranging

Primary Raster Data
- Remote Sensing - getting information about object properties without direct physical contact  A. Satellite remote sensing  B. Aerial photography
- Measuring the amount of electromagnetic radiation reflected, emitted, or scattered from objects
- Three aspects of resolution  A. Spatial - the size of the object that can be resolved  B. Spectral - parts of the spectrum measured  C. Temporal-the frequency with which images are collected for the same area
1. Earth orbiting satellites
2. Stationary satellites

Secondary Raster Data
- Scanned maps and documents are used in GIS as background maps and data stores
- Paper maps, aerial photographs, images  A. Georeferenced  B. Geographic context for vector layers

Secondary Vector Data
Manual digitizing from digitizing tables using stylus or puck with control buttons
Also known as Heads up digitizing.

Point Geocoding
- Geocoding is the process used to plot address data as points on a map.

- Geocoding to the centroids of polygons  A. Election disctrict, zip code, census tract
- Geocoding by street address
A .Also referred as address matching  B. Any supported tabular format  C. One field must specify an address

Data Transfer
- State and local agencies
- Federal government
- Commercial data suppliers
- Intermediate file format  A. Feature Manipulation Engine (FME)
- Direct transfer
- Open Geospatial Consortium (OGC) - a group of vendors, academics, and users interested in the interoperability of geographic systems

Geocoding using addresses
Addresses are matched against a reference data source that contains address information as well as spatial information (feature geometry)

Reference Data source
- Each street segment is mapped as a line object  - Street segment is linked to an attribute table that contains address ranges for the left and right sides of the street

Geocoding in ArcMap
- Create Address Locator in ArcCatalog - Choose address locator style (skeleton of the address locator) - Choose reference layer  A. Streets B. Zip codes - Change address locator properties - Add address locator - View geocoding results - Interactively rematch addresses