Transcript GIS in the Sciences ERTH 4750 (38031) Spatial References and Vector Data Steve Signell, Instructor ([email protected]) Robert Poirier, TA ([email protected]) School of Science Rensselaer Polytechnic Institute Monday,

GIS in the Sciences
ERTH 4750 (38031)
Spatial References and Vector Data
Steve Signell, Instructor ([email protected])
Robert Poirier, TA ([email protected])
School of Science
Rensselaer Polytechnic Institute
Monday, January 27, 2013
Introductions
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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
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Dynamic Maps
http://aprgis.org/argis
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Dynamic Maps
http://poncamap.sig-gis.com/
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Dynamic Maps
http://adkwebmap.com/hamletViz.php
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Introductions
• Name, goals, outcomes
• Classes or experience in:
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Geography, cartography
Other spatial analysis
Web development (html, javascript)
Mathematics
• Smart phones?
• Questions
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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.
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Spatial reference systems
• The earth is round(ish)
• How to represent the
earth (3D) on a 2dimensional surface?
• Maps distort the true
shape of the earth.
• Short Video
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Spatial reference systems
Some definitions:
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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.
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Spatial reference systems
How to represent the earth (3D) on a 2dimensional surface?
Models of the earth
Clark spheroid, 1866
• Sphere
• Ellipsoid
• Geoid
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Spatial reference systems
Example # 1:
• WGS 84, a Geographic Coordinate System
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Spatial reference systems
Example # 2:
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Mercator, a Projected Coordinate System
Preserves bearing
Great for navigation
Distorts area
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Spatial reference systems
Example # 3:
• Gall-Peters, a Projected Coordinate System
• Preserves area
• Distorts shape, bearing.
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Spatial reference systems
Other Examples:
• https://github.com/d3/d3-geo-projection/
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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 EGM96 (Earth Gravitational Model
1996) geoid, revised in 2004
– EPSG: 4326
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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
1940s
– divides the Earth into sixty zones, each a six-degree band of
longitude
– For areas within the contiguous United States, theClarke 1866
ellipsoid[2] was used.
– EPSG: 269xx, e.g. UTM zone 18N =26918
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Spatial reference systems
UTM zones in North America:
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Spatial reference systems
UTM-WGS 84 Demo:
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Questions?
5 min break
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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
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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, GeoTIFF, 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 2D
or coordinate triplets (x,y,z)
when we work in 3D
• Points are best used to
represent objects that are
best described as shapeand 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.
geoJSON)
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/geog486/l1_p8.html
Raster vs. Vector?
Vector
Visualization: https://www.e-education.psu.edu/geog486/l1_p8.html
Raster and vector compared
Raster representation
Vector Representation
Advantages
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Simple data structure
Simple implementation 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)
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