Thursday, 20 January 2011

Some notes on ArcGIS and associated technologies

I’m getting started with ArcGIS so I need to keep some notes. NB: This post is just an aide-mémoire for me as I get started so nothing is covered in any detail.

What is Esri?

Esri is a company providing Geographic Information System (GIS) software and geodatabase management applications. They are based in California and have about 30% of the GIS software market (see http://en.wikipedia.org/wiki/Esri).

What is the APDM?

APDM (ArcGIS Pipeline Data Model) is an open standard for storing geographical data associated with pipelines:

The ArcGIS Pipeline Data Model is designed for storing information pertaining to features found in gathering and transmission pipelines, particularly gas and liquid systems. The APDM was expressly designed for implementation as an ESRI geodatabase for use with ESRI's ArcGIS and ArcSDE® products. A geodatabase is an object-relational construct for storing and managing geographic data as features within an industry-standard relational database management system (RDBMS).” - http://www.apdm.net/

What is ArcSDE?

ArcSDE technology is a core component of ArcGIS Server. It manages spatial data in a relational database management system (RDBMS) and enables it to be accessed by ArcGIS clients.” - http://www.esri.com/software/arcgis/arcsde/index.html

ArcSDE technology serves as the gateway between GIS clients and the RDBMS. It enables you to easily store, access, and manage spatial data within an RDBMS package…

ArcSDE technology is critical when you need to manage long transactions and versioned-based workflows such as

* Support for multiuser editing environments
* Distributed editing
* Federated replicas managed across many RDBMS architectures
* Managing historical archives

The responsibility for defining the specific RDBMS schema used to represent geographic data and for application logic is retained in ArcGIS, which provides the behavior, integrity, and utility of the underlying records.” - http://www.esri.com/software/arcgis/geodatabase/storage-in-an-rdbms.html

What is a geodatabase?

“The geodatabase is the common data storage and management framework for ArcGIS. It combines "geo" (spatial data) with "database" (data repository) to create a central data repository for spatial data storage and management.” - http://www.esri.com/software/arcgis/geodatabase/index.html

Basic terms and concepts

There are four fundamental types upon which geographic representations in a GIS are based:

  • Features (collections or points, lines, and polygons)
    • Representations of things located on or near the surface of the earth.
    • Can be natural (rivers, vegetation, etc).
    • Can be constructions (roads, pipelines, buildings, etc.).
    • Can be subdivisions of land (counties, political divisions, land parcels, etc.).
    • Most commonly represented as points, lines, and polygons.
  • Attributes (descriptive attributes of features)
    • Managed in tables based on simple relational database concepts.
  • Imagery
    • Imagery is managed as a raster data type composed of cells organized in a grid of rows and columns.
    • In addition to the map projection, the coordinate system for a raster dataset includes its cell size and a reference coordinate (usually the upper left or lower left corner of the grid).
    • These properties enable a raster dataset to be described by a series of cell values starting in the upper left row.
    • Each cell location can be located using the reference coordinate, the cell size, and the number of rows and columns.
  • Continuous surfaces (such as elevation)
    • A surface describes an occurrence that has a value for every point on the earth.
    • Surface elevation is a continuous layer of values for ground elevation above mean sea level.
    • Other surface type examples include rainfall, pollution concentration, and sub-surface representations of geological formations.

See the ArcGIS Desktop Help file for further details.

GIS data structures

Features, rasters, attributes, and surfaces are managed using three primary GIS data structures:

  • Feature classes
  • Attribute tables
  • Raster datasets

Map Layer Types GIS Datasets
Features (points, lines, and polygons) Feature classes
Attributes Tables
Imagery Raster datasets
Surfaces

Both features and rasters can be used to provide a number of alternative surface representations:

  • Feature classes (such as contours)
  • Raster-based elevation datasets
  • TINs built from XYZ points and 3D line feature classes

In a GIS datasets hold data about a particular feature collection (for example, roads) that is geographically referenced to the earth's surface. A dataset is a collection of homogeneous features. Most datasets are collections of simple geographic elements.

Users work with geographic data in two fundamental ways:

  • As datasets (homogeneous collections of features, rasters, or attributes)
  • As individual elements (e.g. individual features, rasters, and attribute values) contained within each dataset

Datasets are:

  • The primary inputs and outputs for geoprocessing.
  • Datasets are the primary means for data sharing.

 

See also

There’s some good basic information on GIS systems on the Ordinance Survey website: http://www.ordnancesurvey.co.uk/oswebsite/gisfiles/index.html

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