gis: conceptual reminder franck theeten – bart meganck
TRANSCRIPT
Earth’s Grid System
• Coordinates are usually noted in a Cartesian reference system (with 2 axes, bi-dimensional)
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Earth’s Grid System• Note: the unit for measurements doesn’t
depend from the axes, but from the reference system (it can be degrees, but also meter,
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Earth’s Grid System• Note: the unit for measurements doesn’t
depend from the axes, but from the reference system :– Reference system is combination of
– geoid (real earth surface)– Ellipsoid (mathematical approximation of earth volume)– Shape and position of projection plane– Origin of the 2 axes– Unit of measurement (can be degrees or meter)
the easy part : latitude
• Determine the angle between a position and the earth axis (plane of the equator)
• Not affected by earth rotation• Can be measured by comparing angles
between the soil and the light coming from the Sun
the hard part : longitude
• Value affected by earth rotation• Dependent of time (initially calculated by
clocks aligned on local noon)• Definition of a reference meridian is not
physical but arbitrary– Often linked to political reason and colonial history
• Currently (since the 19th century: Greenwich, UK)• Before, 15th century: Meridian of Tordesillas (separation
between Portuguese and Spanish colonies)
the hard part : longitude
• Value affected by earth rotation• Dependent of time (initially calculated by
clocks aligned on local noon)• Definition of a reference meridian is not
physical but arbitrary– Often linked to political reason and colonial history
• Currently (since the 19th century: Greenwich, UK)• Before, 15th century: Meridian of Tordesillas (separation
between Portuguese and Spanish colonies)
Physical geodesy
• Spirit level /plumb line• Theodolite (angles)• rope/tape measure (distances)• ==> LOCAL gravity
Satellite geodesy
• Geoid is not used• Gravity center of Earth is only point that
sattellites can “feel”, they know nothing about Earth's surface
• Mathematical model for Earth surface : ELLIPSOID
The ELLIPSOID
• Mathematical model for Earth surface• Approximation, never exact !• Not a perfect sphere (1/300 flattening)
Geodetic DATUM
• A geodetic datum is a physical realization of a coordinate system used for describing point locations
• Contain ALL parameters for your reference system
ALWAYS mention the DATUM
• Coordinates without datum are UTTERLY USELESS !!!
• Coordinates without datum are UTTERLY MEANINGLESS !!!
• Coordinates without datum are COMPLETE NONSENSE !!!
Well Known Text notation (WKT)
• A) coordinates
B) Projection
Type « POINT(X Y) »
Ex : -POINT(0 0)
-LINESTRING(0 0, 1 1, 1 2)
-POLYGON (0 0, 1 1, 1 2, 0 0)
- …
PROJCS["NAD83 / UTM Zone 10N", GEOGCS["NAD83", DATUM["North_American_Datum_1983", SPHEROID["GRS 1980",6378137,298.257222101] ], PRIMEM["Greenwich",0], UNIT["degree",0.0174532925199433] ], PROJECTION["Transverse_Mercator"], PARAMETER["latitude_of_origin",0], PARAMETER["central_meridian",-123], PARAMETER["scale_factor",0.9996], PARAMETER["false_easting",500000], PARAMETER["false_northing",0], UNIT["metre",1] ]
Well Known Text for projection
• An important reference:http://spatialreference.org/
• Gives the GDAL expression of some projections that are not embedded by default in tools like QGIS or PostGIS
QGIS: changing the projection settings
Select the projection by categories
Select the projection by name
Plate Carrée
• EPSG:4326 is the default projection of many applicatio, data (GPS, Shapefile) for WGS 84
• Doesn’t prevent local deformation of distances and areas (North appears bigger than equatorial region)
• Plate Carrée
Universal Transverse Mercator
• Looks a “disco ball” (select small rectangular areas to limit deformation of areas, distances and angles to minimal)
UTM Zones
• Interest: unit is meter (distance are easy to express)
• But each zone has its own local origin (not bound to the equator)
• Most of time cannot be applied on world scale layers
UTM Data
• Pretty much use for Raster data – elevation data (SRTM)
– http://srtm.csi.cgiar.org/ – http://www.cgiar-csi.org/data/srtm-90m-digital-elevation-dat
abase-v4-1
– Climatic data– Processed satellite images (LANDSAT)
– https://zulu.ssc.nasa.gov/mrsid
Vector data
• Vector layer:– Mathematical expression of a geographical shape
• points, • lines, • surfaces
– Easy to reproject (parameter of equation)– Already georeferenced– ESRI Shapefile most common data standard for
shapefiles
Vector data (shapefile)
• A shapefile:– Elaborated by ESRI but widely used
• at least 3 (or event better 4) files with the same name but different extensions that must be out in the same folder
Vector data (shapefile)
• ***.shp => the shape (drawn form)• ***.dbf => attribute table (editable in
spreadsheet)• ***.shx=>intermediate index• ***.prj=> Metadata of the projection (WKT format)
Raster data
• Image file or grid used to display matricial data• Need georefencing project• Hard to reproject QGIS can reproject them• Most common standard for
• Images• Altitude data (DEM: Digital Elemevation model:
http://www.cgiar-csi.org/data/elevation/item/45-srtm-90m-digital-elevation-database-v41 )• Climatic data http://www.worldclim.org/download
1) Desktop GIS
• Can produce, update verctor and raster data• Can work without Internet connection• Required to procude the base data (backbone
of GIS)• Example:
– QuantumGIS, Diva-GIS (free)– MapInfo, ESRI ArcINFO (commercial)– …
1) Desktop GIS
• Can produce, update verctor and raster data• Can work without Internet connection• Required to procude the base data (backbone of GIS)• Can not publish data on the Internet• Generally poor metadata support• Example:
– QuantumGIS, Diva-GIS (free)– MapInfo, ESRI ArcINFO (commercial)– …
2) GIS database
• Relational databases with plugins to handle geographical data
• No Graphical interface (but Desktop GIS can connect them and display content)
• Originally worked on vectors, but soince recently can handle rasters and 3-D data
• Powerful to store and modify big vector datasets
• Useful to store maps displayed on the Internet
3) GIS web services
• Services to displayus the maps on the Internet application
• Standardized by Open Geospatial Consortium (a.o. Web Map Server: map are queried via the URL)
• Data are often stored in GIS databases• At this level data can easily be completed by
metadata
3) GIS web services
• Example:– GeoServer (Open source): map server
• http://geoserver.org
– GeoNetwork (OpenSpource, created by FAO): metadata server
• http://geonetwork-opensource.org/
3) GIS web services
• Example:– GeoServer (Open source): map server
• http://geoserver.org
– GeoNetwork (OpenSpource, created by FAO): metadata server
• http://geonetwork-opensource.org/
– OpenLayers: Javascript library to integrate maps in web pages
• http://www.openlayers.org/
Client on the Internet
Server
Scientist computer
3) Typical architecture for Web GIS
Maps and data
Database and query
engines
Maps and search interfaces