INTRODUCTION TO GEOGRAPHY INFORMATION SYSTEMS (GIS)
Q1. Main data sources for GIS
The primary data sources for GIS include; first, web pages. The internet is a platform for accessing GIS data. This GIS data from the internet is in digital format. Again, it is necessary first to evaluate the quality of data found in Metadata before purchasing or downloading it as per the GIS project’s quality standard. Secondly are map sheets. The printed or hard copy maps documents are also sources of GIS data. They vary as per format, content, age, usage, media, scale and complexity but should be legible for providing direct measurements for the desired application. The most current maps sizes are A0, A1, A2, A3 and A4. Global Positioning System (GPS) also contributes to sources of GIS data. Since GPS measures the attributes and positions, GPS data, therefore, is one of the data sources for GIS. This GPS is an electronic system that uses world satellites in locating any position or attribute across the world. GPS has a high accuracy in determining both attributes and positions. There are also data which are generated from Remote Sensing. These data are generally processed then stored in raster data systems. They are easily transferred to raster GIS once they have been transformed into the desired data types. Remote sensing is the most popular source of data in GIS. Its popularity is as a result of several advantages that it offers to the personnel as an element of a GIS system. Some GIS data can be extracted from the Surveying exercise. Surveying is the field-based technique of collecting primary data. Surveyors carry out field trips in capturing data relating to the physical features or phenomena. Data representation from surveying may vary from one researcher to the other. For example, one researcher may label a dam with a dot while the other researcher may represent the same dam using a polygon.
Some of the Canadian sources of road data includes; Natural Resources Canada whose website is www.nrcan.gc.ca (Canada Lands Surveys, 2015), Geobase data source whose website is www.geobase.ca (GeoBase, 2014). Others include Road Network File (RNF) whose website is www5.statcan.gc.ca (Road Network File, 2015), GeoSask data source whose website is www.geosask.ca (GeoSask Metadata, 2016), and the Canadian GIS blog whose website is www.canadiangis.com (Canadian GIS & Geomatics Resources, 2016).
Q2. Slope and Aspect
A slope can be described as the steepness (or gradient) of a terrain unit like a mountain, forest, river, etc., (Dai & Lee, 2012). It is often represented in terms of an angle (degrees) or as a percentage, which can as well be translated into a ration. The slope concept is normally used on topographic surfaces though it can as well be used to analyze other surfaces that are non-topographical in nature. The calculations for slope involve the rate of variation between the rise levels (of two points) and run separation (distance between the two points) (Dai & Lee, 2012). An aspect, on the other hand, can be described as the compass direction or orientation of the slope (Dai & Lee, 2012). Aspect values are often measured in degrees, usually from 0 to 360.
This statement is correct because, physical sizes of various features, depending on the scale, determine whether points should represent objects, lines or areas (Bernhardsen, 2012). A point can be defined as the simplest graphical way of representing an object. It does not apparently necessary have to indicate the measurements involved. Lines are the connection elements that connect two or more points and can be used in representing objects which are defined in one dimension. A line has a beginning (start point) and the end (end point). Polygons are plane objects or figures which are enclosed by three or more straight lines that are intersecting at various points (Bernhardsen, 2012). To achieve such an enclosure, the lines must connect several points and create intersections in that process of linking a point to the other. This fact therefore, means parallel all parallel lines cannot form a polygon. For example, features classified as property boundaries are generally represented by lines such as power cables and telecommunication lines. On the other hand, rivers and roads which also fall under boundary features may be represented by either areas or lines, dependent on the used scale.
Q4. Topology vs. topography
Topology is a term normally used in GIS but if often confused with topography. These two terms have unique meanings. Topography can be defined as the description and study of landscape’s physical features. Topology, on the other hand, is used in describing the relationships that exist between objects (Zlatanova et al. 2014). In GIS, topology has been applied to various uses such as mechanisms that allow phenomena to share geometry, the theory of how features model out in space. It has also been employed to indicate a set of rules for validation purposes, a set of tools for editing integrated features, and a mechanism that enables the navigation between phenomena through the use of their topological connections.
Q5. Ellipsoid and common ellipsoids used in Canada
An ellipsoid is an ellipse which has undergone rotation about its minor (shorter) axis. It is truly a three-dimensional way of representing an ellipse. Ellipsoids (often referred to as spheroids) offer a model of the Earth’s shape (Maling, 2013). Ellipsoid and spheroid are terms that have been applied interchangeably, but it is worth taking into account the consideration that an ellipsoid is a special form or type of spheroid. Some of the commonly used ellipsoids in Canada are; WGS 84 (World Geodetic System of 1984), NAD83 (North American Datum of 1983), and the NAD27 (North American Datum of 1927)
Q6. Raster Model
Raster Data Model is best suited for representing data with the following characteristics; one, continuous spatial objects or features such as terrains, two, data which are containing images, both satellite and aerial images. It is also suitable for representing data which analyses the relationships between continuous features happening on a similar geography such as nutrient levels in a particular setup. Again, Raster correctly describes data that analyses spatial relationships occurring between phenomena such as density, edge contact, interspersion, cost-path among others, and data that deals with the analysis of neighborhood setups, for instance, the precipitation and nutrient concentration within molecular components. In short, Raster model is best suited for continuous data unlike Vector model which is suitable for discrete data.
Q7. NAD27 vs. NAD83
Even though both NAD27 and NAD83 are geodetic systems of referencing, each one is based on separate reference ellipsoids and measurements. NAD27 is based on the 1866’s Clarke ellipsoid, with the reference point being fixed in Kansas. NAD83, on the other hand, is an Earth-centered ellipsoid based on the newly defined GRS80 (Geodetic Reference System of 1980), with its reference point being the earth’s center, other than the surface of the earth (Datum -Census Dictionary, 2015). Spatial data that is based on any of the above reference systems will not coincide with similar spatial data based on the other ellipsoid upon coordinates conversion by the National Transformation software which has been developed by the Geodetic Survey of Canada. Again, differences in position between these two reference systems can be as big as hundreds of metres in certain cases (Datum -Census Dictionary, 2015). Changes in ellipsoids will definitely affect certain longitudinal analyses hence leading to the large variance in these distances.
Q8. UTM system
The Coordinating system of the UTM Zone
UTM projection employs secant, cylindrical and conformal projections, and is generated or derived by placing the cylinder in an east-west position. ‘Transverse’ shows that a cylinder is perpendicular to the one found in standard Mercator, which is placed in a north-south position. ‘Universal’ highlights the fact that it is a world-wide projection. It is made up of 60 zones which are longitudinally wide by 6 degrees (360/6=60), all over the world when positioned north-south (Furuti, 2011). The Coordinating system of the UTM Zone has 6 degrees longitudinal width, which approximates to 672kms on the Equator and tends to narrow towards south and north. Expressing x and y coordinates in metres, the central meridian is given a false easting of 500000m to avoid negative values in a zone. Similarly, the equator is given a false northing of 10,000,000m in order of avoiding negative values within the southern hemisphere (Stefanakis, 2015).
Source: (Ilifee & Lott, 2008).
References
Canada Lands Surveys. (2015, January 16). Retrieved February 17, 2016, from http://www.nrcan.gc.ca/earth-sciences/geomatics/canada-lands-surveys/10780
GeoBase. (2014, May). Retrieved February 17, 2016, from http://ftp2.cits.rncan.gc.ca/pub/geobase/official/nrn_rrn/doc/NRN.pdf
Road Network File (92-500-X). (2015, May 27). Retrieved February 17, 2016, from http://www5.statcan.gc.ca/olc-cel/olc.action?ObjId=92-500-X
GeoSask Metadata. (2016, January 01). Retrieved February 17, 2016, from https://www.geosask.ca/Portal/ptk
Canadian GIS & Geomatics Resources. (2016, January 05). Retrieved February 17, 2016, from http://canadiangis.com/
Dai, F. C., & Lee, C. F. (2012). Landslide characteristics and slope instability modeling using GIS, Lantau Island, Hong Kong. Geomorphology, 42(3), 213-228.
Bernhardsen, T. (2012). Geographic information systems: an introduction. John Wiley & Sons.
Zlatanova, S., Rahman, A. A., & Shi, W. (2014). Topological models and frameworks for 3D spatial objects. Computers & Geosciences, 30(4), 419-428.
Maling, D. H. (2013). Coordinate systems and map projections. Elsevier.
Dana, P. H. (2015). Geodetic Datum Overview. Retrieved February 17, 2016, from http://www.colorado.edu/geography/gcraft/notes/datum/datum_f.html
Ilifee, J., & Lott., R. (2008). Datums and Map Projections: For Remote Sensing, GIS and Surveying. Whittles Publishing.
Datum -Census Dictionary (Canada). (2015, November 27). Retrieved February 18, 2016, from https://www12.statcan.gc.ca/census-recensement/2011/ref/dict/geo017-eng.cfm
Furuti, C. A. (2011). Cartographical Map Projections. Retrieved February 18, 2016, from http://www.progonos.com/furuti/MapProj/Normal/TOC/cartTOC.html
Stefanakis, E. (2015). Web Mercator: the de facto standard, the controversy, and the opportunity. Gogeomatics Magazine. Retrieved February 18, 2016, from http://www.gogeomatics.ca/magazine/web- mercator-the-de-facto- standard-the-controversy-and-the-opportunity.htm#
