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Spatial References and Vector DataSteve Signell, Instructor (signes@rpi.edu)Robert Poirier, TA (poirir@rpi.edu)School of ScienceRensselaer Polytechnic InstituteMonday, January 27, 2013

GIS in the SciencesERTH 4750 (38031) 1IntroductionsInstructor: Steve Signell University of Michigan-BachelorsFerry Beach Ecology School (Maine)Penn State- MastersSUNY-ESF Adirondack Ecological CenterFrontier Spatial, LLCFREE, OPEN SOURCE GIS!

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Reed Dynamic Maps3http://aprgis.org/argis

3Dynamic Maps4http://poncamap.sig-gis.com/

4Dynamic Maps5

http://adkwebmap.com/hamletViz.php 5IntroductionsName, goals, outcomesClasses or experience in:Geography, cartographyOther spatial analysisWeb development (html, javascript)Mathematics Smart phones? Questions6 Spatial data and geoinformationData are representations that can be operated upon by a computer.Spatial data are data that contain positional values.Geospatial data are spatial data that are georeferenced.Metadata are data about data (Who/what/when/where/why)Information is the meaning of data as interpreted by human beings.Geoinformation is information that involves interpretation of spatial data.7 7Spatial reference systemsThe earth is round(ish)How to represent the earth (3D) on a 2-dimensional surface? Maps distort the true shape of the earth.Short Video8

8Spatial reference systemsSome definitions:Thescaleof amapis theratioof a distance on the map to the corresponding distance on the ground.The termshapeis commonly used to refer to the geometric properties of an object or its external boundaryBearing means the angle between a line connecting us and another object, and a north-south line. (i.e. ameridian.[1])Distance, orfarness, is a numerical description of how far apart objects are.Directionis the information contained in the relative position of one point with respect to another point without thedistanceinformation.Areais aquantitythat expresses the extent of atwo-dimensionalsurfaceorshape, orplanar lamina, in theplane.Distortion(orwarping) is the alteration of the original shape (or other characteristic) of something, such as an object, image, sound or waveform.9

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Spatial reference systemsHow to represent the earth (3D) on a 2-dimensional surface? Models of the earthSphereEllipsoidGeoid 10

Clark spheroid, 1866 10Spatial reference systemsExample # 1:WGS 84, a Geographic Coordinate System11

11Spatial reference systemsExample # 2:Mercator, a Projected Coordinate SystemPreserves bearingGreat for navigationDistorts area12

12Spatial reference systemsExample # 3:Gall-Peters, a Projected Coordinate SystemPreserves areaDistorts shape, bearing.13

13Spatial reference systemsOther Examples:https://github.com/d3/d3-geo-projection/14

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Spatial reference systemsGeographic Coordinate Systems:Latitude & LongitudeDegrees, minutes, secondsLatitude: degrees 60 nautical miles apartLongitude: width varies with latitudeMost Popular: World Geodetic System 84(WGS 84)a standard for use incartography,geodesy, navigation.The coordinate origin of WGS 84 is meant to be located at the Earth'scenter of mass; the error is believed to be less than 2cmuses theEGM96(Earth Gravitational Model 1996) geoid, revised in 2004EPSG: 432615

15Spatial reference systemsProjected Coordinate Systems:X,Y coordinatese.g. metersUniversal Tranverse Mercator (UTM)developed by theUnited States Army Corps of Engineersin the 1940sdivides the Earth into sixty zones, each a six-degree band of longitudeFor areas within thecontiguous United States,theClarke1866 ellipsoid[2]was used.EPSG: 269xx, e.g. UTM zone 18N =2691816 16Spatial reference systemsUTM zones in North America:

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17Spatial reference systemsUTM-WGS 84 Demo:

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18Questions?195 min break 19Geographic phenomenaGeographic phenomena are the study objects of a GIS.The real world is complex. A certain spot contains many different phenomena.Different phenomena require different digital representations and multiple representations are possible for a same phenomenon.

PrecipitationWater depthFish CommunityLand use typeCloudsSoil typeWater temperatureElevationTourism DestinationSail Boat Geographic phenomenaA digital representation is a model. It is not the real thing itself.Our representation will never be perfect, some facts will not be found.Essentially, all models are wrong, but some are useful.-- Empirical Model-Building and Response Surfaces (1987)by George E. P. Box and Norman R. Draper Geographic Fields & ObjectsThe next step is to understand how the computer representations can be applied to represent geographic fields and objects

So far we only discussed geographic phenomena, in de following sections we discuss computer representations.

22There are two basic types of geographic fields, discrete and continuousIn a continuous field, the underlying function is assumed to be continuous. Continuity means that all changes in field values are gradual. (for example elevation)Discrete fields cut up the study space in mutually exclusive bounded parts, with all locations in one part having the same field value. (for example land use)

Geographic Fields Continuous FieldsContinuous means that all changes in field values are gradualIn a differentiable field we can measure the changeIn the example on the left, we can measure the gradient (slope) as the change of elevation

Discrete FieldsDiscrete fields cut up the study space in subparts with a clear boundary, with all locations in one part having the same valueTypical examples are land classifications, geological classes, soil types, land use types, crop types or natural vegetation types

Geographic ObjectsObjects are discrete and bounded entitiesThe space between the objects is potentially empty or undetermined

Three rocks (objects), in between no rocks (empty) Geographic ObjectsThe position of an object in space is determined by a combination of one or more of the following parametersLocation (where is it?)Shape (what form?)Size (how big?)Orientation (direction)

A bridge is an object, with a location, shape, size (length of the bridge) and a direction (maybe north south) Geographic ObjectsWe usually do not study objects in isolation (a single object) but whole collections of objectsObserve that collections of objects can be interesting phenomena at a higher aggregation level:Forest plots form forests Parcels form blocks and blocks form suburbsStreams, brooks and rivers form a river drainage systemWe can study each individual tree, or the combination of trees, as one object

BoundariesTwo basic types of boundaries:

Crisp boundariesA crisp boundary is one that can be determined with almost arbitrary precisionAs a general rule of thumb, crisp boundaries are more common in man-made phenomena

Fuzzy boundariesFuzzy boundaries contrast with crisp boundaries in that the boundary is not a precise line, but rather an area of transition

Raster & Vector Data TypesComputer representations can be divided into two groups: raster and vector-based representations

So far we only discussed geographic phenomena, in de following sections we discuss computer representations.

32Raster = any type of digital image represented by reducible and enlargeable gridsconsists of rows and columns of cells, with each cell storing a single valueCan have multiple bands, e.g. red, green, blueJPEGJPEG, TIFF, GeoTIFF, ESRI GRIDRaster data types

Different geographical features are expressed by different types of geometry:PointsLines (or polylines)Polygons

Why are they called vectors?Vector data types

PointsPoints are defined as single coordinate pairs (x,y) when we work in 2D or coordinate triplets (x,y,z) when we work in 3DPoints are best used to represent objects that are best described as shape- and sizeless, single-locality features

Points representing trees along a road 35LinesUsed to represent one-dimensional objects (e.g., roads, railroads, canals, rivers)Line is defined by 2 end nodes and 0-n internal nodes to define the shape of the line.An internal node or vertex is like a point that only serves to define the line

Begin nodeEnd nodeVertexLine or arc 36Areas (polygons)When area objects are stored using a vector approach, the usual technique is to apply a boundary modelThe area is defined by the boundary of the area

You store the boundary of the area 37Each geometry can be linked to a row in a database (attribute table). Vector Attributes

Or.geometry can be linked to attributes in other formats (e.g. geoJSON) Vector Attributes

Raster geographic phenomenon for which, for every point in the study area, a value can be determined (e.g., temperature, land cover, barometric pressure and elevation)

Vector well distinguishable, discrete, bounded entities. The space between them is potentially empty (e.g., buildings, rivers)

Raster vs. Vector?

RasterRaster vs. Vector?

Visualization: https://www.e-education.psu.edu/geog486/l1_p8.html

VectorRaster vs. Vector?Visualization: https://www.e-education.psu.edu/geog486/l1_p8.html Raster and vector comparedRaster representationVector RepresentationAdvantagesSimple data structureSimple implementation of overlaysEfficient for image processingMay better represent fuzzy boundariesAdapts well to scale changesAllows representing networksAllows easy association with attribute dataMay better represent discrete boundariesDisadvantagesLess compact data structureCell boundaries independent of feature boundariesComplex data structureOverlay more difficult to implementInefficient for image processing 43Vectors and QGISLayer Properties: symbology, metadata, styling

44Vectors and QGIS

Geoprocessing: 45Vectors and QGISEditing:

46For ThursdayReadings: (links at course web site: http://tw.rpi.edu/web/Courses/GIScience/2014)Vector Attribute Data (QGIS User Guide) Vector Properties (QGIS User Guide)

Install QGIS on your laptop! (qgis.org)Download example data (will post to course website ASAP)47