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What is a coordinate system?
Coordinate systems enable geographic datasets to use common locations for integration.
A coordinate system is a reference system used to represent the locations of geographic
features, imagery, and observations such as GPS locations within a common geographic
framework.
Each coordinate system is defined by:
• Its measurement framework which is either geographic (in which spherical
coordinates are measured from the earth's center) or planimetric (in which the earth's
coordinates are projected onto a two-dimensional planar surface).
• Unit of measurement (typically feet or meters for projected coordinate systems or
decimal degrees for latitude-longitude).
• The definition of the map projection for projected coordinate systems.
• Other measurement system properties such as a spheroid of reference, a datum, and
projection parameters like one or more standard parallels, a central meridian, and
possible shifts in the x- and y-directions.
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Coordinates
Defining the location of a point in the chosen coordination.
Coordinates can be geodetic (
,h), space (X,Y,Z) or planar (x,y or
N,E). Location can be defined with the latitude, longitude and the
height. Height can ortometric (H), height from the ellipsoid (h) or
normal height (H).
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Coordinate system
Coordination
Datum
Coordinates
Relationship between coordinate system, coordination,
coordinates and the datum
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Coordinate system: ETRS89 (European Terrestrial Reference System)
• 3-D coordinate reference system
• Origin: Earth’s center of mass
• Ellipsoid: Global GRS80 (Geodetic reference system)
•
Projections: ETRS-TM35FIN,
• Universal Transverse Mercator (UTM)
• Transverse cylindrical, conformal
• One projection zone
• Center meridian 27 E (cut)
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• Projections: Gauss-Kruger
• ETRS-GK, KKJ
• Transverse cylindrical, conformal
• Several projection zones (tangential)
i.e., several central meridians
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Kartastokoordinaattijärjestelmä (KKJ)
• Ellipsoid: Hayford 1924
• 6 projection zones, 3 wide
• Yhtenäiskoordinaatisto, zone 3
Abbrev
zone
Center meridian
East coordinate on
center meridian
KKJ0
0
18°
500 000 m
KKJ1
1
21°
1500 000 m
KKJ2
2
24°
2500 000 m
KKJ3
3
27°
3500 000 m
KKJ4
4
30°
4500 000 m
KKJ5
5
33°
5500 000 m
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Coordinations (coordinate reference frame):
• ETRS-TM35FIN - EUREF-FIN (UTM/GRS80)
• ETRS-GKn (Gauss-Kruger/GRS80), n=center
meridian
• KKJ (Gauss-Kruger/Hayford)
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Some Definitions
Projection: a projection is the mathematical transformation of
Earth’s 3-d surface to a 2-d planar surface (a map)
Datum: a point of reference from which distance measurements
are made (local vs. Earth centered)
Spatial Reference: includes info on the projection, projection
parameters, ellipsoid, datum, units of measure, and other
parameters.
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Types of coordinate systems
There are two common types of coordinate systems used in GIS:
• A global or spherical coordinate system such as latitude-longitude. These are
often referred to as geographic coordinate systems (e.g. WGS84).
• A projected coordinate system based on a map projection such as transverse
Mercator, Albers equal area, or Robinson, all of which (along with numerous
other map projection models) provide various mechanisms to project maps of
the earth's spherical surface onto a two-dimensional Cartesian coordinate plane
(e.g. EUREF-FIN). Projected coordinate systems are sometimes referred to as
map projections.
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A geographic coordinate system (GCS)
is a method for describing the position of a geographic location on the Earth's
surface using spherical measures of latitude and longitude. These are
measures of the angles (in degrees) from the center of the Earth to a point on
the Earth's surface. A GCS is often incorrectly called a datum, but a datum is
only one part of a GCS. A GCS includes an angular unit of measure, a prime
meridian, and a datum (based on a spheroid).
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The shape and size of a geographic coordinate system's surface is defined by a sphere or
spheroid. Although the earth is best represented by a spheroid, the earth is sometimes
treated as a sphere to make mathematical calculations easier. The assumption that the earth
is a sphere is possible for small-scale maps (smaller than 1:5,000,000). At this scale, the
difference between a sphere and a spheroid is not detectable on a map. However, to
maintain accuracy for larger-scale maps (scales of 1:1,000,000 or larger), a spheroid is
necessary to represent the shape of the earth. Between those scales, choosing to use a sphere
or spheroid will depend on the map's purpose and the accuracy of the data.
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Mean Sea Level, GPS, and the Geoid
Traditionally topographic or bathymetric elevation is related to
sea level. Geodesists once believed that the sea was in balance
with the earth's gravity and formed a perfectly regular figure.
Mean sea level (MSL) is usually described as a tidal datum that is
the arithmetic mean of hourly water elevations observed over a
specific 19-year cycle.
Goce
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Relationships between the geoid and the ellipsoid
The accuracy of GPS height measurements depends on several factors but the
most crucial one is the "imperfection" of the earth's shape.
Height can be measured in two ways:
1.
Height (h) above the reference
ellipsoid (used by the GPS)
that approximates the earth's
surface.
2.
Orthometric
(traditional)
height (H) is the height above
an imaginary surface called
the geoid, which is determined
by the earth's gravity and
approximated by MSL.
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GPS elevations have no physical meaning and
they don’t correlate in anyway with the
height of the sea level to which traditional
elevations are usually measured. (Positio
2/2008)
The terms geographic coordinate system and
datum are often used interchangeably. Datum is an
older term. The concept of a datum is included
within a geographic coordinate system.
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DATUMS
While a spheroid approximates the shape of the earth, a 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. Whenever you
change the datum, or more correctly, the geographic coordinate system, the coordinate
values of your data will change.
Satellite data has provided geodesists with new measurements to define the best earthfitting spheroid, which relates coordinates to the earth’s center of mass. An earth
centered, or geocentric, datum uses the earth’s center of mass as the origin. The
most recently developed and widely used datum is WGS 1984. It serves as the
framework for locational measurement worldwide.
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A local datum aligns its spheroid to closely fit the earth’s surface in a particular area.
• The point, origin of the datum, on the surface of the spheroid is matched to a particular
position on the surface of the earth.
• The coordinates of the origin point are fixed, all other points are calculated from it.
• The coordinate system origin of a local datum is not at the center of the earth. The
center of the spheroid of a local datum is offset from the earth’s center.
• NAD 1927 (North America) and the European Datum of 1950 (ED 1950) are local
datums.
• Because a local datum aligns its spheroid so closely to a particular area on the earth’s
surface, it’s not suitable for use outside the area for which it was designed.
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Projected coordinate systems are any coordinate system designed
for a flat surface, such as a printed map or a computer screen.
• 2D and 3D Cartesian coordinate systems provide the mechanism
for describing the geographic location and shape of features
using x and y values.
• The Cartesian coordinate system uses two axes: one horizontal
(x), representing east-west, and one vertical (y), representing
north-south.
• In a projected coordinate system, locations are identified by x,y
coordinates on a grid, with the origin at the center of the grid.
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• A projected coordinate system is defined on a flat, twodimensional surface.
• Unlike a geographic coordinate system, a projected coordinate
system has constant lengths, angles, and areas across the two
dimensions.
• However, all map projections representing the earth's surface as
a flat map, create distortions in some aspect of distance, area,
shape, or direction
• A projected coordinate system is always based on a geographic
coordinate system that is based on a sphere or spheroid.
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Map projections
Because the earth is round and maps are flat, getting information from a curved
surface to a flat one involves a mathematical formula called a map projection, or
simply a projection. This process of flattening the earth will cause distortions in
one or more of the following spatial properties:
• Distance
• Area
• Shape
• Direction
No projection can preserve all these properties; as a result, all flat maps are
distorted to some degree. Fortunately, you can choose from many different map
projections.
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Map projection classifications
Map projections can be generally classified according to what spatial attribute they
preserve.
• Equal area projections preserve area. Many thematic maps use an equal area
projection. Maps of the United States commonly use the Albers Equal Area Conic
projection.
• Conformal projections preserve shape and are useful for navigational charts and
weather maps. Shape is preserved for small areas, but the shape of a large area,
such as a continent, will be significantly distorted. The Lambert Conformal Conic
and Mercator projections are common conformal projections.
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• Equidistant projections preserve distances, but no projection can preserve
distances from all points to all other points. Instead, distance can be held true
from one point (or a few points) to all other points or along all meridians or
parallels. If you will be using your map to find features that are within a certain
distance of other features, you should use an equidistant map projection.
• Azimuthal projections preserve direction from one point to all other points.
This quality can be combined with equal area, conformal, and equidistant
projections, as in the Lambert Equal Area Azimuthal and the Azimuthal
Equidistant projections.
• Other projections minimize overall distortion but don't preserve any of the four
spatial properties of area, shape, distance, and direction. The Robinson
projection, for example, is neither equal area nor conformal but is aesthetically
pleasing and useful for general mapping.
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Conic projections
The most simple conic projection is tangent to the globe along a line of latitude. This line
is called the standard parallel. The meridians are projected onto the conical surface,
meeting at the apex, or point, of the cone. Parallel lines of latitude are projected onto the
cone as rings. The cone is then cut along any meridian to produce the final conic
projection, which has straight converging lines for meridians and concentric circular arcs
for parallels. The meridian opposite the cut line becomes the central meridian.
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Cylindrical projections
Like conic projections, cylindrical projections can also have tangent or secant cases. The
Mercator projection is one of the most common cylindrical projections, and the equator
is usually its line of tangency. Meridians are geometrically projected onto the cylindrical
surface, and parallels are mathematically projected. This produces graticular angles of 90
degrees. The cylinder is cut along any meridian to produce the final cylindrical
projection. The meridians are equally spaced, while the spacing between parallel lines of
latitude increases toward the poles. This projection is conformal and displays true
direction along straight lines.
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Planar projections
Planar projections project map data onto a flat surface touching the globe. A
planar projection is also known as an azimuthal projection or a zenithal
projection. This type of projection is usually tangent to the globe at one point
but may be secant, also. The point of contact may be the North Pole, the South
Pole, a point on the equator, or any point in between.
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Reasons for using a projected coordinate system
The following are some reasons for using a projected coordinate system:
• You want to make accurate measurements from your map and be sure that the spatial
analysis options you use calculate distance correctly. Latitude-longitude is a good
system for storing spatial data but not as good for viewing, querying, or analyzing maps.
Degrees of latitude and longitude are not consistent units of measure for area, shape,
distance, and direction.
• You are making a map in which you want to preserve one or more of these properties:
area, shape, distance, and direction.
• You are making a small-scale map such as a national or world map. With a small-scale
map, your choice of map projection determines the overall appearance of the map. For
example, with some projections, lines of latitude and longitude will appear curved; with
others, they will appear straight.
• Your organization mandates using a particular projected coordinate system for all maps.
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http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html
http://store.usgs.gov/b2c_usgs/b2c/usgs/netfile?file=//igskahcigssap05/mod
/storefiles/PDF/16573.pdf
http://www.uff.br/mapprojections/mp_en.html
http://www.jhlabs.com/java/maps/proj/index.html
http://www.fgi.fi/fgi/sites/default/files/publications/gltiedote/GLtiedote30.p
df
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Some topics covered
• Default geodatabase
• Downloading Data
• Using ArcCatalog
• Coordinates, Spatial Reference
• Creating an ArcMap Project
• Adding Data Layers
• Renaming Data Layers and Data Frames
• Some other basic features in ArcMap…
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Downloading Data , Using ArcCatalog and Defining Spatial
Reference Properties
•
Start ArcMap
•
Set up a Work Folder … save your files and work here.
•
ArcCatalog – ’Connect to folder’ i.e., find your Work Folder
•
Download data from Paituli
1.
NLS, general Map 1:250000 (MLL, yleiskartta)
!Not everything!
1.
NLS, general Map 1:1000000 (MLL, yleiskartta)
• Read the terms of use
• From Noppa
1.
•
10milj_kp.zip (Bedrock map 1:10000000)
Unzip downloaded data to your Work Folder
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ArcMap- Home folder
• Home folder - folder location where your map document is stored
• Access in the Catalog window
• Changing the Home folder location – Options/Home Folder
The default geodatabase
• Default geodatabase, is the home location for the spatial content of your
map.
• It is used for adding and saving resulting datasets created by various editing
and geoprocessing operations.
• E.g. exporting features from a layer, the data will be saved by default in the
map's default geodatabase.
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Changing default geodatabase
•
the Catalog window (right-click the geodatabase you want to use as the
default for your current document and choose Make Default Geodatabase)
•
The Map Document Properties dialog box (click File > Map Document
Properties).
Access to the default geodatabase is available from the Catalog window menu,
and as a shortcut on the Add Data and Save As dialog boxes
Selecting default geodatabase
•
New map document -select a default geodatabase-by default Default.gdb
•
Existing map document-read-only-the default geodatabase of the map
document.
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