Understanding Maps & GIS

Scholars GeoPortal > Understanding Maps & GIS

GIS Terminology

For more GIS Terminology, see the ESRI GIS Dictionary.

Aerial – An image taken of the earth’s surface taken from a platform flying within the earth’s atmosphere, typically an aircraft.

Area data representation – Portions of the earth’s surface that are represented by polygons (see Polygons). Each polygon would represent an area (such as a field, water body or building boundary).

Area of Interest (AOI) – The extent used to define a focus area for a map, database, air photo or project. For example, your area of interest may be a school campus and all of the buildings, parking lots, sports fields and bike lanes that fall within that area.

Attributes – Information about a feature (point, line or polygon) that is typically stored in a table and linked to the spatial data. This information can typically be mapped.

Base Maps – A map on which GIS data layers are placed onto or registered to. Typically, base maps contain reference information such as place names, street names, landmarks and/or political boundaries. These help provide references and added background information to your GeoPortal search and results. The Base Map detail is not downloadable from the Scholars GeoPortal.

Bitmap (BMP) –An image format where one or more bits represent each pixel. The number of bits per pixel determines the range of grey tones or colours that the image can display.

Boundary File – A polygon representation of a geographic areas. For example, a boundary file consist may be of census tracts or political boundaries.

Database – A collection of datasets that are typically stored together and organized by relationships. The datasets are linked to each other through common fields found in different tables.

Download – Retrieving data off of an internet site or database. Downloading data or files will allow a user to save the material on their personal computer or storage device (USB).

Download Format – A specific format can be chosen for the data being downloaded. Formats may be GIS specific, such as .shp (shape file) or grid (raster). Image formats include JPEG, BMP, TIFF, etc.

Draw an Extent – Draw a bounding shape around your area of interest. For example, if you are interested in the Niagara Region, draw a rectangle around the edges of Niagara.

Extract Data – See Download

Geodatabase – A database or file structure used primarily to store, query, and manipulate spatial data. Geodatabases are similar to standard databases; however, they contain a spatial component (i.e. x and y coordinate information).

GEOTiff – An image format that allows for storing georeferencing information such as projection or coordinate systems in the metadata.

Geocoding – A GIS operation for converting street addresses into spatial data that can be displayed as features on a map, usually by referencing address information from a street segment data layer.

Geographic Information Systems (GIS) – arrangement of computer hardware, software, and geographic data for integrating, analyzing, and visualizing data. From this new relationships, patterns, and trends may be identified.  applications used for the viewing, integrating and manipulation of spatial data. A GIS represents the data as layers which can visualized and then analyzed with the system tools.

Grid – In cartography, any network of parallel and perpendicular lines superimposed on a map and used for reference, such as latitude and longitude. Grids are usually referred to by the map projection or coordinate system they represent, such as Universal Transverse Mercator (UTM) grid. For Grid in data models, see Raster.

Hybrid base map – Base maps that contain a combination of imagery and vector data (such as road networks, labels, etc).

Identify – A tool used to select a feature in your map viewer that pulls up the attribute information for that feature. The attribute data will provide information about the selected feature. For example, selecting a building with the identify tool will bring up information related to that building.

Imagery – A representation or description of a scene, typically either an aerial image, satellite image or raster image. In all cases, the image is made up from a group of cells that create the scene.

Inset Map – A smaller map set within a larger map. The inset map is used to either show the study area in greater detail, or to show the study area in connection with the surrounding area (in smaller detail).

JPEG – An image format that allows for lossy compression (reduces image file size by removing data).

Layer – The visual representation of a single data theme in a digital map environment, such as, roads, vegetation, buildings, or water bodies, with their corresponding attributes.

Legend – The legend describes the type and symbology of features located on a map. Legends should be located on most maps, especially when symbols cannot be immediately interpreted by the map user.

Line data representation – A line data representation is a connection of points (with x and y coordinates) that create either a straight or curved path. For example, a road could be represented by a line, with x and y coordinates located at the start and end of the road, as well as any location where the road changes direction or angle.

Metadata – Information that describes the characteristics of the data such as descriptions of content, origin, date, quality, file size, location, etc.

Open Data – Data that is freely available and accessible through online data resources (portals and websites). Such data can often be used without restriction or copyright issues.

Orthophotography – Aerial photography in its digital form that is free from any distortions relating to camera tilting and ground relief changes. The orthophoto has a consistent scale like a map.

PNG – A bitmapped image format that allows for lossless data compression (no information is lost in the compression).

Place – A populated place name.

Points of Interest (POI) – A specific point location or series of points that someone may find useful or important. For example, a point of interest may be your home while a series of points may be restaurants you enjoy going to.

Point data representation – A location on the earth’s surface that is represented by x and y coordinates. For example, a school may be represented by a point (depending on the scale of the map).

Polygon data representation – A closed shape made up of x and y coordinates where all coordinate pairs are unique except for the first and last, allowing the shape to be closed. For example, a lake may be represented by a polygon.

Projection – The process of converting earth’s round surface to a flat surface. Projections typically rely on systems of intersecting lines that create a graticule (grid of intersecting x and y lines) for plane creation. Projections typically distort distance, area, shape or any combination of these.

Raster – A data format that contains an array of equally sized cells arranged in rows and columns. Each cell contains an attribute value and location coordinates. Raster files can be made up of single or multiple bands.

Satellite Imagery – Images captured of the earth’s surface from satellites orbiting the planet. 

Scale – The ratio or relationship between a distance or area on a map and the corresponding distance or area on the earth’s surface. Scale can be expressed as a representative fraction, verbal scale or scale bar.

Series – A collection of related themes, such as boundaries, or a collection of different themes packaged together by the creator, such a CanMap Route Logistics.

Shaded Relief – A raster image that shows changes in elevation using light and shadows on terrain from a given angle and altitude of the sun.

Terrain – An area of land having a particular characteristic (e.g. rocky terrain).

Theme – a set of related geographic features such as roads, contours, or rivers. All features in a theme share the same coordinate system, are located within a common geographic extent, and have the same attributes.

TIFF – An image format that includes additional geometry information, such as dimensions, as tags in a header file.

Topographic – Vertical and horizontal representations of features on the earth’s surface. For example, a map showing contour lines could be considered topographic in nature.

Vector – A coordinate-based data model that represents geographic features such as points, lines, and polygons. For example, a road network could be represented by a series of points and lines, saved as a vector file format.

Zoom – The ability to magnify or reduce maps or images on-screen.

References

Geographical information systems(1999). In Longley P. (Ed.), (2nd ed.). New York: Wiley.

Lo, C. P. (2002). In Yeung A. K. W. (Ed.), Concepts and techniques of geographic information systems. Upper Saddle River, N.J.: Prentice Hall.

Schuurman, N. (2004). GIS: A short introduction. Malden, Mass.: Blackwell Pub.

Tang, W. (2005). In Selwood J. (Ed.), Spatial portals: Gateways to geographic information (1st ed.). Redlands, Calif.: ESRI Press.

Tomlinson, R. F. (2011). Thinking about GIS: Geographic information system planning for managers (4th ed.). Redlands, Calif.: Esri Press.

Coordinate Systems and Projections

Coordinate systems: A coordinate system is a reference system used to represent the locations of geographic features within a common framework. There are two types of coordinate systems used in GIS: geographic coordinate systems and projected coordinate systems.

Geographic coordinate systems: Geographic coordinate systems use a three-dimensional spherical surface to represent the earth and define locations on it, which are referenced by longitude and latitude values.

Because the earth is not completely round, it isn’t possible to base a coordinate system on a perfect sphere. Therefore a variety of “spheroids” are used instead, each of which is more accurate in a particular part of the world. In North America, the most popular spheroid is known as GRS 1980. Another spheroid which is used is WGS 1984.

Going even further, spheroids can have datums associated with them, which incorporate local variations in elevation, since the earth is not really smooth. In North America, the most popular datums are NAD 1983 (North American Datum; based on the GRS 1980 spheroid) and WGS 1984 (World Geodetic System; based on the WGS 1984 spheroid).

Geographic Coordinate Systems available for download in Scholars GeoPortal include WGS 1984, NAD 1983, and NAD 1927.

Also note that, while not directly available, the CSRS (Canadian Spatial Reference System) is another geographic coordinate system which underlies many of the Canadian projected coordinate systems offered in the portal

Projected coordinate systems: Projection refers to the process of representing the earth’s round surface as a flat surface (a piece of paper, or your computer screen). Projected coordinate systems, or projections, are always based on a geographic coordinate system (described above).

There are many different types of projections, and each one of necessity distorts the representation of the earth in some way. When selecting a projection for your data, it is important to be aware of what distortions are in place – certain types of distortion are more acceptable for some purposes than for others.

The following projections, including descriptions, are available in Scholars GeoPortal:

Lambert Conformal Conic
Type: Conic
Properties: Conformal – all angles at any point are preserved. Maintains shapes for small areas (i.e. large scale maps), but distorts the size of large areas.
Common uses: mapping Canada and the US; mapping equatorial/mid-latitute areas; mapping at large and medium scales. This projection is commonly used by the Canadian government.
Examples – the following projections offered by the GeoPortal are examples of the Lambert Conformal Conic projection:

  • Canada Lambert Conformal Conic (based on NAD 1983)
  • Statistics Canada Lambert (developed by Statistics Canada; based on NAD 1983)
  • Statistics Canada Lamber CSRS (developed by Statistics Canada; based on NAD 1983/CSRS)
  • Ontario MNR Lambert (developed by the Ministry of Natural Resources; based on NAD 1983)
  • Ontario MNR Lamber CSRS (developed by the Ministry of Natural Resources; based on NAD 1983/CSRS)

Transverse Mercator
Type: Cylindrical
Properties: Conformal – all angles at any point are preserved. Maintains shape for small areas – specifically, in zones only a few degrees wide in east-west extent.
Common uses: large scale topographic map series, NTS and USGS maps
Examples – the following projections offered by the GeoPortal are examples of the Transverse Mercator projection:

  • UTM (Universal Transverse Mercator; based on NAD 1983). Note: you must pick the “zone” which applies for the area you wish to download. Here is a UTM Zone Map for reference (Source: Statistics Canada)

A note on the Web Mercator projection. This is offered by the GeoPortal because it has become the projection commonly used for web mapping – in Google Maps, ArcGIS online, and others. It has also been referred to as “Popular Visualization Pseudo Mercator”. However, this projection is typically not recommended for use in GIS analysis, as it is not a “true” mercator projection. If you’re interested in learning more, we recommend this article (PDF).

For more information on projections and coordinate systems in general, we recommend the ESRI ArcGIS Resource Center’s section on Map Projections.

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