Midterm Review (Mod 1-5) Flashcards
What is a Datum?
A datum defines the position of a spheroid (approximates the shape of the earth) relative to the centre of the earth. Datums provide a frame of reference for measuring locations on the surface of the earth (origin and orientation of latitude/longitude)
What is a Coordinate System?
Way to locate and display data correctly on the earth’s surface and relative to one another
Reference framework that defines the position of features in either two or three-dimensional space
What is the difference between a Local Datum and a Earth-Centered Datum?
Local Datum – aligns spheroid to closely fit earth’s surface in a
particular area, uses an origin point. For example NAD 1927 or
ED 1950
Earth-centered datum (geocentric) uses the earth’s center of
mass as the origin, for example WGS 1984 is the most recently
developed and widely used datum which serves as the
framework for locational measurement worldwide.
What are the two types of vertical datums?
- Gravity-based (geoidal) coordinate system (more
commonly used). - Ellipsoidal (spheroidal) coordinate systems.
What are the differences between a Geographic Coordinate System and a Projected Coordinate System?
Geographic Coordinate Systems
* Based on a three-dimensional ellipsoidal or spherical model of the earth.
* Commonly has units in decimal degrees measuring degrees of latitude-longitude.
* Measurements of angle, not distance.
Projected Coordinate Systems
* Based on mathematical formulas to translate the world into flat (planar) space
* This adjustment is called a map projection
* Uses a Geographic coordinate system and a map projection
* All map projections introduce spatial distortion
* Length-based unit of measure, origin point for measurements
What is a Geographic Datum Transformation?
- Method to reconcile the misalignment of the datum
- Often done in conjunction with a coordinate system projection
- Permanently applied projection using data processing tool
(Project) or - Temporarily applied within the map pane/data view “on the
fly” (defaults to US transformations)
What are the two types of transformation methods?
- Equation-based transformation methods
- Grid-based transformation methods (very common in North America)
Explain the differences between the “Define Projection” tool and the “Project” tool.
Define Projection Tool (Projections and Transformations Toolset)
* Overwrites the coordinate system information (map projection and
datum) stored within a dataset
* Intended for datasets that have an unknown or incorrect coordinate
system defined
* Only updates (defines) the existing information, it does not modify any
geometry
Project Tool (Projections and Transformations Toolset)
* Projects spatial data (not rasters) from one coordinate system to another
What is a Geodatabase?
An ArcGIS geodatabase is a collection of geographic datasets of various types
held in a common file system folder, or a multiuser relational DBMS
- Newest data structure and primary data format used for editing and management, ESRI is trying to move away from the Shapefile (but the shapefile is still WIDELY used in industries)
Information model for representing and managing geographic information
- Series of tables holding feature classes and attributes
- Data objects add real world behaviour, rules and tools for working with spatial relationships (eg. Topology, networks)
- Also includes working with shapefiles, CAD files, TINs, grids, imagery, GML and other data
sources
Three primary dataset types: Feature classes, raster datasets and tables
Name the 3 types of Geodatabases.
File Geodatabase (.gdb)
* Collection of various types of GIS datasets held in a single file system folder and has no total size limit
* Each dataset within has a default size limit of 1TB but can be increased to 4 or 256TB for extremely large image datasets
* Designed to support network and terrain datasets and relationship classes
* Supports multiple editing on different datasets
* Supports multiple readers on single dataset
* Can only view within ArcCatalog or ArcCatalogPane
* Uses about 1/3 of the feature geometry storage used by Shapefiles
Mobile Geodatabase (.geodatabase)
* All the datasets that belong to one mobile geodatabase are contained in an SQLite database that is stored in a single file
* Size limit is 2TB
* Ability to create, display, and query GIS data
* Ability to edit simple data such as point, multipoint, line geometries & polygons
* A portable geodatabase that works across operating systems, good for mobile app development
* Uses an efficient data structure that is optimized for performance and reduced cost, complexity, and administration
Enterprise geodatabase
* Collection of various types of GIS datasets held as tables in a relational database
* Relational database systems such as Oracle, Microsoft SQL Server, PostgreSQL or IBM DB2
* Size limits determined by DBMS vendor
* Supports many simultaneous editors and readers on individual datasets
* Relational database support for GIS data management
* Benefits of a relational database for scalability, reliability, security backup and data integrity
UNOFFICIAL 4th: Personal Geodatabase (.mdb) – not supported in ArcGIS Pro
* Microsoft Access Database
* Can store, query and manage both spatial and nonspatial data
* Total size limit of 2GB
* One user at a time can edit database
* This is the older technology of the file geodatabase and is not as stable
What is the difference between a join and a relate?
Unlike joining tables, relating tables simply defines a relationship between
two tables
* The associated data isn’t appended to the layer’s attribute table like it is with a join. Instead, you can access the related data through selected features or records in your layer or table
Name 5 different data types that can be used in columns of a table.
Number of supported attribute column types used to hold and manage
attributes
* Numbers: short or long integers, floats, doubles
* Text: alphanumeric characters
* Date: date and time data
* BLOBs: binary large objects used to store documents, photos (not in a shapefile)
* GUIDs: registry style unique strings consisting of 36 characters enclosed in curly brackets (not in a shapefile)
Name the 7 types of feature classes.
- Points
- Lines
- Polygons
- Annotation
- Dimensions
- Multipoints
- Multipatches
What are “m-values”?
Linear measures (m-values), measurements along line features
* Support for this data type is referred to as linear referencing
* The process of geolocating events that occur along a measurement system is referred to as dynamic segmentation
* Eg. Engineering companies building pipelines will often use this to reference the route of the pipeline from the start to the end using Metre or Kilometre markers
What is the difference between a domain and a subtype?
Domain:
Used to constrain the values allowed in any particular attribute for a table or
feature class, the field will not accept a value that is not in that domain
Subtypes
Subtypes are a subset of features in a feature class, or objects in a table
that share the same attributes
* A method to categorize data (red text in slides)
