GIS in the Sciences ERTH 4750 38031 Spatial

GIS in the Sciences ERTH 4750 (38031) Spatial References and Vector Data Steve Signell, Instructor (signes@rpi. edu) Robert Poirier, TA (poirir@rpi. edu) School of Science Rensselaer Polytechnic Institute Monday, January 27, 2013

Introductions • • Instructor: Steve Signell University of Michigan-Bachelors Ferry Beach Ecology School (Maine) Penn State- Masters SUNY-ESF Adirondack Ecological Center Frontier Spatial, LLC FREE, OPEN SOURCE GIS! Reed 2

Dynamic Maps http: //aprgis. org/argis 3

Dynamic Maps http: //poncamap. sig-gis. com/ 4

Dynamic Maps http: //adkwebmap. com/hamlet. Viz. php 5

Introductions • Name, goals, outcomes • Classes or experience in: – – Geography, cartography Other spatial analysis Web development (html, javascript) Mathematics • Smart phones? • Questions 6

Spatial data and geoinformation • Data 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

Spatial reference systems • The earth is round(ish) • How to represent the earth (3 D) on a 2 dimensional surface? • Maps distort the true shape of the earth. • Short Video 8

Spatial reference systems Some definitions: • • The scale of a map is the ratio of a distance on the map to the corresponding distance on the ground. The term shape is commonly used to refer to the geometric properties of an object or its external boundary Bearing means the angle between a line connecting us and another object, and a north-south line. (i. e. a meridian. [1]) Distance, or farness, is a numerical description of how far apart objects are. Direction is the information contained in the relative position of one point with respect to another point without the distance information. Area is a quantity that expresses the extent of a twodimensional surface or shape, or planar lamina, in the plane. Distortion (or warping) is the alteration of the original shape (or other characteristic) of something, such as an object, image, sound or waveform. 9

Spatial reference systems How to represent the earth (3 D) on a 2 dimensional surface? Models of the earth Clark spheroid, 1866 • Sphere • Ellipsoid • Geoid 10

Spatial reference systems Example # 1: • WGS 84, a Geographic Coordinate System 11

Spatial reference systems Example # 2: • • Mercator, a Projected Coordinate System Preserves bearing Great for navigation Distorts area 12

Spatial reference systems Example # 3: • Gall-Peters, a Projected Coordinate System • Preserves area • Distorts shape, bearing. 13

Spatial reference systems Other Examples: • https: //github. com/d 3 -geo-projection/ 14

Spatial reference systems Geographic Coordinate Systems: • Latitude & Longitude – Degrees, minutes, seconds – Latitude: degrees 60 nautical miles apart – Longitude: width varies with latitude • Most Popular: World Geodetic System 84 (WGS 84) – a standard for use in cartography, geodesy, navigation. – The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm – uses the EGM 96 (Earth Gravitational Model 1996) geoid, revised in 2004 – EPSG: 4326 15

Spatial reference systems Projected Coordinate Systems: • X, Y coordinates – e. g. meters • Universal Tranverse Mercator (UTM) – developed by the United States Army Corps of Engineers in the 1940 s – divides the Earth into sixty zones, each a six-degree band of longitude – For areas within the contiguous United States, the. Clarke 1866 ellipsoid[2] was used. – EPSG: 269 xx, e. g. UTM zone 18 N =26918 16

Spatial reference systems UTM zones in North America: 17

Spatial reference systems UTM-WGS 84 Demo: 18

Questions? 5 min break 19

Geographic phenomena Land use type Clouds Precipitation • Geographic 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. Water temperature Soil type Water depth Elevation Sail Boat Fish Community Tourism Destination

Geographic phenomena • A 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 & Objects The next step is to understand how the computer representations can be applied to represent geographic fields and objects

Geographic Fields There are two basic types of geographic fields, discrete and continuous – In 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)

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

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

Geographic Objects • Objects are discrete and bounded entities – The space between the objects is potentially ‘empty’ or ‘undetermined’ Three rocks (objects), in between no rocks (empty)

Geographic Objects The position of an object in space is determined by a combination of one or more of the following parameters – – A bridge is an object, with a location, shape, size (length of the bridge) and a direction (maybe north –south) Location (where is it? ) Shape (what form? ) Size (how big? ) Orientation (direction)

Geographic Objects • We usually do not study objects in isolation (a single object) but whole collections of objects • Observe that collections of objects can be interesting phenomena at a higher aggregation level: We can study each individual tree, or the combination of trees, as one object – Forest plots form forests – Parcels form blocks and blocks form suburbs – Streams, brooks and rivers form a river drainage system

Boundaries Two basic types of boundaries:

Crisp boundaries • A crisp boundary is one that can be determined with almost arbitrary precision – As a general rule of thumb, crisp boundaries are more common in man-made phenomena

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

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

Raster data types • Raster = any type of digital image represented by reducible and enlargeable grids • consists of rows and columns of cells, with each cell storing a single value • Can have multiple ‘bands’, e. g. red, green, blue JPEG • JPEG, TIFF, Geo. TIFF, ESRI GRID

Vector data types Different geographical features are expressed by different types of geometry: • Points • Lines (or polylines) • Polygons Why are they called vectors?

Points • Points are defined as single coordinate pairs (x, y) when we work in 2 D or coordinate triplets (x, y, z) when we work in 3 D • Points are best used to represent objects that are best described as shape- and sizeless, single-locality features Points representing trees along a road

Lines Begin node – Used to represent onedimensional 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 Vertex Line or arc End node

Areas (polygons) – When area objects are stored using a vector approach, the usual technique is to apply a boundary model – The area is defined by the boundary of the area You store the boundary of the area

Vector Attributes Each 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. geo. JSON)

Raster vs. Vector? 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? Raster Visualization: https: //www. e-education. psu. edu/geog 486/l 1_p 8. html

Raster vs. Vector? Vector Visualization: https: //www. e-education. psu. edu/geog 486/l 1_p 8. html

Raster and vector compared Raster representation Vector Representation Advantages • • Simple data structure Simplementation of overlays Efficient for image processing May better represent fuzzy boundaries • Adapts well to scale changes • Allows representing networks • Allows easy association with attribute data • May better represent discrete boundaries Disadvantages • Less compact data structure • Cell boundaries independent of feature boundaries • Complex data structure • Overlay more difficult to implement • Inefficient for image processing

Vectors and QGIS Layer Properties: symbology, metadata, styling

Vectors and QGIS Geoprocessing:

Vectors and QGIS Editing:

For Thursday Readings: (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