Georeferencing Principles and Approaches

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Georeferencing Principles and Approaches
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1. Introduction

• Georeferencing refers to the process of
  registering the maps to a common coordinate
  systems
• Georeference refers to an unique location.
  Everybody understands the same thing. There is no
  confusion. E.g. Location of Chicago Latitude 41
  49 N and longitude 87 54" W. It remains
  unchanged through time.
• These may be simple names (UTM), or metric based
  on measurements
• In metric geo-reference by knowing the
  measurements of two locations you can compute the
  distance.

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2. Placenames

• Simple georeference, E.g. Hinsdale, IL
• You can use direction, distance with placenanes.
  Can also use the qualifiers like near or between.
• Placenames may become lost through time

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3. Postal addresses

• Postal addresses provide good georeferences to
  human dwellings
• Natural features cannot be referenced in postal
  address system
• Zip codes may also be changed

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4. Linear referencing

• Linear systems are widely used in transportation
  systems and emergencies
• A accident is reported by position measured along
  a linear distance
• These may be sufficient for some applications

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5. Cadastral and Public Land System

• It is based on a prime meridian North South and
  ranges that are equally spaced apart.
• If the Earth were flat there would be a good
  system of georeferencing

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6. Measuring Earth Lat Long

• The earth is flat. A map projection converts the
  3d earth to 2D map
• The diameter along the equator is 1 in 300 part
  longer than diameter along the pole
• North pole, South pole, Rotational axis and
  Equatorial plane provide the reference to
  location on the earth.
• Lat long is called the Geographic projection
  system. Lat goes from 0 to 90N and 90 S and
  Longitude goes from 0 to 180 E and 180 W.
• Lines of latitude or longitude are equally spaced
  apart only at the equator. Can you explain why?
• Given the latitude and longitude, it is possible
  to determine the distance between the two points

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Earth As Spheroid
Earth is a Spheroid. A spheroid is defined by
either the semimajor axis, a, and the semiminor
axis, b, or by a and the flattening. The
flattening is the difference in length between
the two axes expressed as a fraction or a
decimal. The flattening, f, is f (a - b) / a
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Datum
Spheroid approximates the shape of the
earth. Datum defines the position of the spheroid
relative to the center of the earth. A datum
provides a frame of reference for measuring
locations on the surface of the earth. It defines
the origin and orientation of latitude and
longitude lines.
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A point is referenced by its longitude and
latitude values. Longitude and latitude are
angles measured from the earths center to a
point on the earths surface.
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7. Projection Systems and Coordinates

• Map projection transforms earths surface
  identified by the latitude and longitude into a
  flat Cartesian (X,Y) co-ordinate system.
• A position of a point in a Cartesian coordinate
  system has origin (datum) and coordinates
  (projection system)
• Can two geographic datasets differ both in datum
  and in projection systems?

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Projected Coordinate System
A projected coordinate system is always based on
a geographic coordinate system that is based on a
sphere or spheroid. In a projected coordinate
system, locations are identified by x,y
coordinates on a grid, with the origin at the
center of the grid. Each position has two values
that reference it to that central location. One
specifies its horizontal position and the other
its vertical position. The two values are called
the x-coordinate and y-coordinate. Using this
notation, the coordinates at the origin are x 0
and y 0.
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Equal area Projections
Equal area projections preserve the area of
displayed features. To do this, the other
propertiesshape, angle, and scaleare distorted.
In equal area projections, the meridians and
parallels may not intersect at right angles. In
some instances, especially maps of smaller
regions, shapes are not obviously distorted, and
distinguishing an equal area projection from a
conformal projection is difficult unless
documented or measured.
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Equidistant Projections
Equidistant maps preserve the distances between
certain points. Scale is not maintained correctly
by any projection throughout an entire map
however, there are, in most cases, one or more
lines on a map along which scale is maintained
correctly. Such distances are said to be true.
For example, in the Sinusoidal projection, the
equator and all parallels are their true lengths.
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True Distance Projections
The shortest route between two points on a
curved surface such as the earth is along the
spherical equivalent of a straight line on a flat
surface. That is the great circle on which the
two points lie. Truedirection, or azimuthal,
projections maintain some of the great circle
arcs, giving the directions or azimuths of all
points on the map correctly with respect to the
center. Some true-direction projections are also
conformal, equal area, or equidistant.
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Conformal
Conformal projections preserve local shape. To
preserve individual angles describing the spatial
relationships, a conformal projection must show
the perpendicular graticule lines intersecting at
90-degree angles on the map.
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7. Projection Systems and Coordinates

• Map projections are of three types
• 1. Cylindrical Like wrapping a paper cylinder
  around the earth and noting where the positions
  on earth surface is projected on this cylindrical
  paper.
• 2. Azimuthal or planner Like wrapping a plane
  sheet of paper
• 3. Conic Like wrapping a conical paper around
  the earth
• UTM (universal Transverse Mercator) is a
  cylindrical projection system which has zones
  with 6 degrees of longitudinal width. Easy to
  recognize as there are six digit integer followed
  by seven digit integer.
• UTM is in meters, therefore easy to compute
  distance.
• US states have their own specialized systems.
  E.g. Texas has Lambert Conformal Conic System,
  while Hawaii has UTM

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Planar
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Conic
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Cylindrical
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8. Converting Georefernces

• Geographic projection system is attractive
  because you can determine their lat long or UTM
  directly at the touch of a button.
• There are routines that allow projection from one
  system to the other based on projection system
  properties
• GIS softwares allow projection from one system to
  the other
• Georeferencing is the process to assigning a
  common coordinate to the map layers.
• You can use a particular projection system to
  preserve a certain property.
• You can go from one projection system to the
  another based on known properties.

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GEOGRAPHIC TRANSFORMATIONS
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Three Parameter Method
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Seven Parameter method
S Scale Factor
Scale 3 Change in new 3 original
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9. Summary

• Georeferencing is the process of assigning common
  coordinate system to your map layers.
• Georeferncing allows us to measure distance, area
  and perform analysis.
• Georeferencing enables to locate phenomena
  unambiguously and accurately.
• Accuracy or perfection of geo-referencing is tied
  up with spatial resolution or scale.

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Quick review
Geographic entity
Geographic Data

• Map Scale 3 types
• Map essentials Title scale, 3D, 2l, 1P
• Representation Capture variability in space and
  time, Gather information and build over time
• What to represent and how to represent
  purposefully
• Toblers Law
• Representation in computer is binary

Non-spatial (Attributes)
Spatial
Geometric
Nominal Ordinal Interval Ratio Cyclic
Points Lines Polygons
Discrete
Fields
Raster
Vector
Data representation, Attribute Structure,
Georefereincing