Geographic Information Systems using QGIS 1 - INTRODUCTION
Generalities A GIS (Geographic Information System) consists of: -Computer hardware -Computer software - Digital Data
Generalities GIS softwares are aimed to store, handle, modify, update, view, query, analyze, interpolate, print, import and export geographic information
Generalities Georeference Using GIS, every geographic feature has to be characterized by its location on the Earth s surface Coordinates metric (X, Y) or geographic (Lat, Lon) + Coordinate System (Geographic / Projected, Method, Geodetic parameters)
Coordinate Reference Systems To properly georeference a data layer, besides coordinates a Coordinate Reference System (CRS) (i.e. the equation describing the geometry used to calculate coordinates) is needed. Two types of CRS can be used: - Geographic - Cartographic
Geographic CRSs Geographic CRSs allow to define the position of objects on the Earth surface using their Latitude and Longitude (angular values). A geographic CRS basically is the description of the assumed Earth geometry (sphere / ellipsoid / geoid; radius / semiaxes etc.); i.e. geodetic information.
Cartographic CRSs Cartographic CRSs allow to define the position of objects on the Earth surface using distances from reference meridians and parallels calculated on their proiection. Besides geodetic data, the formula used to project the Earth on the plane / cylinder / cone is needed.
Dynamic storage Features can be easily inserted, deleted, modifyied, updated or deleted Paper : Once created, it is almost impossible to modify a map
Cartographic systems Data sets can be re-proiected in every possible cartographic system Paper : To change the cartographic system it is necessary to redraw the map (generally with distortion)
Cartographic systems It is easy to overlay information deriving from data sets having different cartographic systems Paper : To overlay data with different cartographic system it is necessary to redraw the map (generally with distortion)
Scale Features are independent from scale Paper : The represented features depend on the scale
Scale The scale of representation can be freely chosen and easily modified Paper : Only fixed scales are available; change of scale is difficult and not very precise
Scale Symbols, labels and annotations are automatically rescaled when enlarging/reducing the scale Paper : To properly rescale the representation it is necessary to draw the whole map
Map sheets The size and shape of the map can be freely chosen Paper : Map sheets have fixed boundaries; generally, they require manual mosaicing
Background map The background map can be widely personalized Paper : It is difficult and time consuming to change the background map
Symbology It is easy and fast to change the symbols attributed to every feature Paper : It is almost impossible to change the symbols
Symbology It is easy and fast to change the symbols attributed to every feature Paper : It is almost impossible to change the symbols
Transparency The transparency of each data layer can be easily set Paper : It is extremely complicate to draw applying a proper transparency
Legend Legends are automatically created Paper : Legends are to be drawn manually
Labels Labels can be automatically applied to features Paper : Labels are to be written manually
Annotations Annotations can be freely moved, added, removed, rescaled, rotated, modified etc. Paper : Annotations are to be managed manually
Drawing No special skill is needed to create and draw symbols Paper : Only skilled designers are able to create nice maps
Drawing No special skill is needed to create and draw symbols Paper : Only skilled designers are able to create nice maps
Layouts Scale bars, north arrows etc. can be automatically placed and rescaled. Paper : Everything is to be manually drawn every time
Layouts Custom layouts can be created, stored and applied to every representation Paper : Layouts are to be manually drawn every time
I / O Data can be easily imported and/or exported from/to other sw Paper : I/O operations are almost impossible, beside simple print and scan
Calculations Mathematical calculations can be carried out on single and multiple data sets Paper : No calculation is allowed, beside simple manual geometric measurements
Statistics Statistic analyses can be carried out on single and multiple data sets Paper : No statistics can be automatically carried out
Queries A simple click allows to know all the properties of a point, even though they are contained in different data layers Paper : Several maps are to be perused to collect multiple information on a single point
Data extraction It is easy to automatically extract a subset of data from a layer Paper : Only manual redraw of each subset is possible
Precision Every feature can be stored with extremely high precision (mm on the ground) Paper : The precision of the representation depends upon its scale
Interpolation Sparse data can be quickly interpolated using different methods Paper : Manual interpolation os extremely time consuming and (generally) non very accurate
Reclassification It is easy and fast to reclassify features according to their properties Paper : Reclassification requires to draw the whole derived map
Derived maps It is easy and fast to create derived maps (e.g. slope angle or aspect from elevation) Paper : To derive maps is extremely time consuming and non very accurate
Remote sensing IT is possible to use a GIS to make many RS quantitative analyses Paper : No quantitative analysys of RS imagery is possible.
3D views It is easy and fast to create perspective views (once stored the distribution of elevations) Paper : Rare drawing skills are needed to create 3D views (generally much less accurate)
Cost Many GIS package are quite expensive but many others are free Paper : No expensive tools are needed
Education It takes time to learn how to use GIS packages: they are quite complex, offering a very wide set of tools Paper : No particularly complex knowledge is needed to draw a map (but only those who are able to draw very well will obtain good results)
Data Structures Data structures VECTOR RASTER TABLE
Data Structures Vector Very precise but complicated; needs topology POINTS LINES POLYGONS
Data Structures Vector Vector data are made up of points GIS do not allow to mix up different geometry types (points, polylines, poligons) Choosing the geometry type to use depends on scale, convenience and aim of the work. Accuracy depends upon scale
Points A dataset of points is a random list of couples of coordinates (X, Y), each with its unique feature identifier. For 3D data, also the Z value is stored Data Structures
Lines Data Structures A dataset of lines is a random list of ordered arrays of points (individuating the nodes of the sequence of segments); each array has its unique feature identifier.
Polygons Data Structures A dataset of polygons is a random list of ordered arrays of lines closing on thmselves (individuating the boundaries of the polygon); each array has its unique feature identifier.
Rasters Simple but not very precise; A raster file is a grid (i.e. a regular rectangular matrix of cells) e.g. a digital photo Data Structures
Data Structures Rasters Raster maps are good to show continually varying information The size of cells (pixels) determines the spatial resolution and the geometric accuracy Increase of spatial resolution determines the creation of nuch larger files
Data Structures Rasters Also Raster data are to be georeferenced to be properly used To georeference a raster file, only one couple of coordinates (X, Y or Lat, Lon) is needed, related to one of its corners, plus the grid geometry (dimesion of cells, number of rows and columns).
Tables Data Structures Contain ancillary information Tables contain the attributes of geographic objects (Points, Lines, Polygons and Raster cells). They cannot be georeferenced (i.e. they have no coordinates).
Text (alphanumeric) Data Types Number Nominal (e.g. an international phone code) Ordinal (e.g. MCS earthquake intensity) Cardinal With a conventional zero (e.g. temperature) With a real zero (e.g. population)
Most diffuse GIS packages Open source: QGIS Rather easy, basic to mid level Will be used for this GIS course Grass Quite complex, mid to advanced level Kind of upgrade of QGIS aimed at data analyses SAGA Really complex, very advanced level Aimed to advanced statistical and geostatistical analyses Commercial: ArcGIS Complex, basic to very advanced level Mostly used for advanced data management
GIS and external plugins Many plugins are available, both commercial and free, to carry out specific procedures using QGIS or ArcGIS or other GIS packages. Be always sure to use only plugins created for the brand and release of SW you have installed.
QGIS releases QGIS (Quantum GIS) is frequently updated, introducing new features and correcting bugs. This course will be based on QGIS 2.18 Las Palmas de Gran Canaria Different releases may show slight differences in appearance.
QGIS Download: SW package (Windows, Mac OS X, Linux, BSD, Android) Documentation (User Guide + Training manual) Sample data (at least, the Alaska dataset) from http://qgis.org Download and install the latest version! (at present, 2.8.1 Wien ; but they upgrade often )
QGIS QGIS software allows to translate its commands in many languages: anyhow, I strongly suggest to maintain the English locale. Some documentation is available in various languages too.