ITS488-2.2 Map projection · LastLecture& • Spaaldatamodel:& –...
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ITS488 – Geographic Informa5on System Teerayut Horanont, PhD Sirindhorn Interna5onal Ins5tute of Technology (SIIT) Thammasat University E: [email protected] 22 Map projec5on 2014 08 20 8/19/14 ITS488 1
Transcript of ITS488-2.2 Map projection · LastLecture& • Spaaldatamodel:& –...
ITS488 – Geographic Informa5on System
Teerayut Horanont, PhD Sirindhorn Interna5onal Ins5tute of Technology (SIIT) Thammasat University E: [email protected]
2-‐2 Map projec5on 2014 08 20
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Updates
• Class homepage at: – hQp://www2.siit.tu.ac.th/teerayut/
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Last Lecture
• Spa$al data model: – you learned about fundamental elements of GIS data.
– Vector VS Raster – File format – Input GIS data
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Today’s Outline
• We will learn one of the most fundamental and important aspect of all maps, as all maps require the transforma5on.
• We will see some examples of each map projec5on.
• How to apply map projec5on for applica5on development.
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What are map projec5ons?
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Source: hQps://support.google.com/earth/answer/148111?hl=en
What are map projec5ons?
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Source: hQps://support.google.com/earth/answer/148111?hl=en
“A map projec5on is a mathema5cal formula used to transfer all or part of the curved surface of the earth onto the flat surface of a map. ”
Fact and you should know
• There are hundreds of different map projec5ons.
• The process of flaQening the earth causes distor5ons in one or more of the following spa5al proper5es: – Distance – Area – Shape – Direc5on
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Fact and you should know
• No projec5on can preserve all these proper5es. (distance, area, shape, direc.on)
• Some map projec5ons minimize distor5on in one property at the expense of another, while others strive to balance the overall distor5on.
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You have to decide which proper5es are most important (for your applica5on) and choose a projec5on that suits your needs.
What are map projec5ons?
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The point P at 75°E 55°N mapped by the polar aspect of the azimuthal projec5on. On the lek, the Earth rests on the projec5on plane; on the right, the point already projected.
Projec5on parameters
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Top: Round data is described with meridians, parallels, and la5tude-‐longitude values. Bo/om: Flat data is described with x,y units. (x,y can be lat, lon also) Projec5on parameters use in both kinds of descrip5ons.
Projec5on parameters
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Concepts Angular parameters ·∙ Central meridian ·∙ La5tude of origin ·∙ Standard parallel ·∙ Central parallel (equator) Linear parameters ·∙ eas5ng ·∙ northing ·∙ wes5ng ·∙ southing ·∙ Scale factor
You can search for defini5ons from hQp://support.esri.com/en/knowledgebase/GISDic5onary/search
Projec5on types Many types of map projec5ons have been devised to suit par5cular purposes. Common map projec5ons are classified according to the projec5on surface used
1. conic
2. cylindrical 3. planar
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Conic (tangent)
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A cone is placed over a globe. The cone and globe meet along a la5tude line. This is the standard parallel. The cone is cut along the line of longitude that is
opposite the central meridian and flaQened into a plane.
Conic (secant)
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A cone is placed over a globe but cuts through the surface. The cone and globe meet along two la5tude lines. These are the standard parallels. The cone is cut along the line of longitude that is opposite the central meridian and flaQened
into a plane.
Cylindrical
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A cylinder is placed over a globe. The cylinder can touch the globe along a line of la5tude (normal case), a line of longitude (transverse case), or
another line (oblique case).
Planar
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A plane is placed over a globe. The plane can touch the globe at the pole (polar case), the equator (equatorial case), or another line
(oblique case).
Polar aspect (different perspec5ves)
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Azimuthal, or planar projec5ons can have different perspec5ve points. The gnomonic projec5on's point is at the center of the globe. The opposite side of the globe from the point of contact is used for a stereographic projec5on. The perspec5ve point for an orthographic
projec5on is at infinity.
Map scale
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Map scale is the ra5o of a distance on the map to the corresponding distance on the ground (real world).
Map scale
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Map scales may be expressed in words (a lexical scale), as a ra5o, or as a frac5on. Examples are: 'one cen5meter to one hundred meters' 1:10,000 or 1/10,000 'one inch to one mile’ 1:63,360 or 1/63,360 'one cen5meter to one thousand kilometers' 1:100,000,000 or 1/100,000,000 (The ra5o would usually be abbreviated to 1:100M)
Map scale
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Map Projec5ons and Distor5on
Map projec5ons can be generally classified according to what spa5al aQribute they preserve. 1) Equal area projec$ons preserve area.
The equal-‐area projec5ons maintain correct propor5ons in the sizes of areas on the globe and corresponding areas on the projected grid.
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Equal-‐area projec5ons are preferred for small-‐scale thema5c mapping, especially when map viewers are expected to compare sizes of area features like countries and con5nents.
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Cylindrical equal-‐area projec5on of the world; standard parallel at 40°N.
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No5ce: the areas each one occupies are equivalent.
Cylindrical Equal Area projec5on
Map Projec5ons and Distor5on
2) Conformal projec$ons preserve shape. Shape is preserved for small areas, but the shape of a large area, such as a con5nent, will be significantly distorted. The Lambert Conformal Conic and Mercator projec5ons are common conformal projec5ons.
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vary substan5ally in size, but are all the same circular shape.
Map Projec5ons and Distor5on
3) Equidistant map projec$on preserve distances but no projec5on 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.
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Map Projec5ons and Distor5on
4) Azimuthal projec$ons preserve direc5on(azimuths) from one point to all other points on the map. This is a good projec5on for uses like plorng airline connec5ons from one airport to all others.
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Display all great circles as straight lines. See how the ellipses ploQed on the gnomonic projec5on shown above vary in both size
and shape, but are all oriented toward the center of the projec5on?
What projec5ons you want to use?
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What is a coordinate system?
• Coordinate systems enable geographic datasets to use common loca5ons for integra5on.
• A coordinate system is a reference system used to represent the loca5ons of geographic features, imagery, and observa5ons within a common geographic framework.
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Each coordinate system is defined by the following:
• 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)
• Units of measurement (typically meters for projected coordinate systems or decimal degrees for la5tude-‐longitude)
• Other measurement system proper5es such as a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shiks in the x-‐ and y-‐direc5ons
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Datum
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The datum was assigned a la5tude and longitude based on astronomical measurements, as well as a height above an ideal chosen ellipsoid. Hence, the datum consists of both the parameters for the ellipse and the chosen ini5al point.
Datum • The ellipsoid is a mathema5cal
model that describes the shape of the earth. �
• A datum defines the posi5on of the ellipsoid rela5ve to the center of the earth
� • Provides a frame of reference
for measuring loca5ons on the surface of the earth
� • Aligns its spheroid to closely fit
the earth’s surface in a par5cular area
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Datum shik
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Types of coordinate systems
1) Geographic coordinate systems (GCS). – A global or spherical coordinate system – A point is referenced by its longitude and la5tude values
2) A projected coordinate system (PCS)
– PCS is defined on a flat, two-‐dimensional surface.
– such as universal transverse Mercator (UTM)
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Geographic transforma5ons
• If two datasets are not referenced to the same geographic coordinate system, you may need to perform a geographic transforma5on.
• This is a well-‐defined mathema5cal method to convert coordinates between two geographic coordinate systems.
• there are several hundred predefined geographic transforma5ons that you can access.
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EPSG Geode5c Parameter Dataset
• The EPSG Geode5c Parameter Dataset is a structured dataset of Coordinate Reference Systems and Coordinate Transforma5ons, accessible through this online registry (www.epsg-‐registry.org)
• Sample # WGS 84 – <4326> +proj=longlat +datum=WGS84 +no_defs <> # Indian 1975 / UTM zone 48N – <24048> +proj=utm +zone=48 +a=6377276.345
+b=6356075.41314024 +towgs84=210,814,289,0,0,0,0 +units=m +no_defs <>
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What wrong with my map??
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What wrong with my map??
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Map projec5on for programming Example: using proj4js library // include the library <script src="lib/proj4js-‐combined.js"></script> // crea5ng source and des5na5on Proj4js objects // once ini5alized, these may be re-‐used as oken as needed var source = new Proj4js.Proj('EPSG:4326'); //source coordinates will be in Longitude/La5tude, WGS84 var dest = new Proj4js.Proj('EPSG:3785'); //des5na5on coordinates in meters, global spherical mercators projec5on, see hQp://spa5alreference.org/ref/epsg/3785/
// transforming point coordinates var p = new Proj4js.Point(-‐76.0,45.0); //any object will do as long as it has 'x' and 'y' proper5es Proj4js.transform(source, dest, p); //do the transforma5on. x and y are modified in place
//p.x and p.y are now EPSG:3785 in meters
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Ques$on! 1) What is Thailand (Or your country) projected coordinate system? (A Common use one) 2) What is EPSG parameters of above projected coordinate system? (write down EPSG number and parameters) 3) Which datum transforma5ons is suitable for Thailand (Or your country) ? 4) Please explain Degree-‐based formats
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Open for ques5ons
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