Basics
- Object Vs. Field
- Geoid＋sphere/ellipsoid=datum
- Geoid is dynamic and ellipsoid is more theoretical, we can align both of
them
- If the alignment is local, where elliposid surface is closely fit to
the geoid at a particular location on earth’s services ⇒Local
datum(vertical)
- defines elevation (height) relative to a local reference
surface, often the mean sea level (MSL)
- Or Geocentric where the ellipsoid is aligned with the center of
the earth ⇒Geocentric datum(horizontal) E.g., WGS84
- defines the horizontal position (latitude and longitude) on
the Earth's surface based on a reference ellipsoid that is
centered at or near the Earth's center of mass.
- How we choose to align the ellipsoid to the geoid defines a datum
W1
Components of GIS
1. 2 (or 3) dimensional spatial component (here: horizontal plane)
2. An (often implicit) temporal component (here: z-axis)
3. A thematic component (here: symbology)
,Reference systems
1. Established sets of rules for the measurement of phenomena and their
relations
2. Constrains inter-subjective interpretation and reference to these phenomena
⇒meausrement symbols obtain an inter-subjective meaning and become
reproducible
3. Reference systems constrain operations one can apply to data
4. Adding up values requires the same reference system
Attribute reference systems
- Interpretations of signs into a domain of measurement ⇒E.g., interpretation of
“1” into a length(meter) ⇒fixed by convention (think about the prototype
meter)
- E.g., Meter scale for measuring lengths and distances
Temporal reference systems
- Can be linear (e.g., “unix time” seconds since 1 January 1970) or circular
(weekdays, months, hours of a day…)
- ISO 8601 data elements and interchange format, based on the Gregorian
calendar(Greenwich mean time)
- Forms the basis of xsd: dateTime
CRS(coordinate reference systems)
, - Datums: A datum is the information that fixes a coordinate system to some
reference object (the Earth, Geoid)
- Horizontal datums:
- Ellipsoid model of Earth with geodetic (geographic
coordinates)
- Map projection(projected coordinates)
- Vertical datums(fixing height). E.g., ellipsoidal height
Geodetic datums for geographic coordinates
- Horizontal(geodetic) datum consist of:
- Reference Elliposid(defined by major and minor axis lengths)
- …fixed relative to the earth via a standard position(fundamental point)
- …some standard orientation(of minor axis, aligned with poles)
- …and the location of prime meridian子午線(0 degree, e.g., Greenwich)
- Different “realizations” of datums in different years (e.g., WHS84=world
geodetic system of 1984)
Geographic datum shifts: positional errors
- Shifts of coordinate positions (in meters) between different realizations of the
North American Datum (NAD)
- Note: errors can be more than 80 meters in CA ⇒if you have a dataset based
on NAD 83 but you measure them as NAD 27, it will cause errors
Map projections
- How to get from an ellipsoid to the plane? Not without information loss!
- Three types of planar projections:
- Gnomic(from center)
- Stereographic(from opposite side)
- Orthographic(parallel projection)
- This projection is often used in mapping polar regions but can be used
for any location on the earth’s surface (in which case they are called
oblique planar
projections).
- Cylindrical projection
- A cylindrical map projection maps the earth surface onto a map rolled
into a cylinder (which can then be flattened into a plane). The cylinder
, can touch the surface of the earth along a single line of tangency (a
tangent case), or two lines of tangency(a secant case)
- Distortion is minimized along the tangent or secant lines and
increases as the distance from these lines increases
- E.g., UTM⇒Mercator, cylindrical equal-area
- Conical projection
- A conical map projection maps the earth surface onto a map rolled into
a cone. Like the cylindrical projection, the cone can touch the surface
of the earth along a single line of tangency (a tangent case),or two
lines of tangency(a secant case)
- E.g., Conical projections are also popular PCS’ in European maps such
as Europe Albers Equal Area Conic and Europe Lambert Conformal
Conic.
-
- Defined by
- class(cylindrical, conical or azimuthal)
- Point of secancy(tangent or secant)
- aspect(normal, transverse or oblique)
- Distortion property(equal area equidistant or conformal, or no
property)
,CRS transformations
- To homogenize geodata, involving 3 principal approaches:
- Geocentric transformations (via geocentric cartesian coordinates)
- Geographic transformations (via coordinate offsets)
, - 2D plane(affine,polynomial) transformations
Geodata access and retrieval
Interoperability
- Technical interoperability is the ability of different software systems to
communicate and interact via shared interfaces
- Technical interoperability for data and software can be solved via
- Standardized data formats
- Standardized web services
- Standardized interfaces (APIs)
- Semantic interoperability is the ability to search, find, and make use of data
from different sources (providers, organizations, measurements)
Basic geodata formats: raster and vector
- Raster encodes space into georeferenced cells
, - Vector encodes space into georeferenced points, lines and areas
- Important data formats:
- Vector: shp(shapefile,ESRI),WKT(well known text), GML(Geography
markup language), GeoJSON
- Raster: GeoTIFF, netCDF, IMG,GRID(ESRI)
Web service architecture and RESTful APIS
Typical software architectures
- Client-server
- 3-tier
- Presentation: user interface
- Application logic: calculations, decisions, transformations etc
- Data: storage and retrieval of data
-
Distribute tiers on the Web
- Web applications
- N-tier architecture: the presentation layer is a web page/site
- Many possible requests and server responses
- Many alternative implementations with varying requirements
What are Web API’s?
- API=application programming interface
- Exchange of data between a web service and a program
- Use of web services from within a program
How to access API ⇒http requests
- Http requests are fired against an endpoint)computing device communicating
back and forth)
-
,Restful services (representational state transfer)
- Uniform interface:
- Http GET request to receive data
- Http POST request to push data
- Stateless: no client is stored on the server between requests. Each request
from any client contains all the information necessary to service the request
- Individual resources are identified using URIs in Web-based REST systems
- Makes services easy to access via standard clients(URI and HTTP) as
opposed to SOAP(simple object access protocol) protocol
Geosocial media: foursquare and Open street map(OSM)
Problems
- Rate limits: the amount of data you can get is usually limited
- Read the social media platforms for details
- Low quantities are usually free, large quantities can still be obtained
with fees
- Frequency and amount of queries are restricted(denial of service)
- API changes: companies are free to change syntax or stop services
- Your code may need to be adapted once services are changed.
- Legal: restrictions in data usage for certain purposes
- Data quality: social media data is far from an unbiased sample
Linked open data(LOD) principles: link data on the web, not (only) documents
- Use URI to denote things
- Use HTTP URI so that these things can be referred to and looked up
(“dereferenced”) by people and user agents
- Provide useful information about the thing when its URL is dereferenced,
leveraging standards such as RDF, SPARQL
Open geographic information standards
- Open standards are a result of consensus delivered by international
organizations
- For geographic information
- ISO standards
- ISO 19115:2003 defines the schema required for describing
geographic information and services. It provides information
about the identification, the extent, the quality, the spatial and
temporal schema, spatial reference, and distribution of digital
geographic data.
- Open geospatial consortium(OGC) specifications
- For web standards
- W3C(html,RDF,XML,HTTP…)
- Spatial data on the Web Best practices
- Currently, most geodata services do not yet adhere to such RESTful/LOD
standards⇒still with SOAP standards
, OGC simple features(ISO19125)
- Geometry model where geometries are described by straight line segments or
planar interpolation between point sets (point, linestring, polygon)...
- Encoded as either Geography markup(GML) or Well-known text(WKT)
- WKT(Well known text)
- WKT is an OGC and ISO standard for representing geometries,
coordinate reference systems, and transformations
WKT vs GML vs Geojson
- WKT:
- A text-based format for representing vector geometry (points, lines,
polygons, etc.).
- Compact and human-readable (E.g., POINT(30 10) LINESTRING(30
10, 10…)
- No support for complex data structures like multi-geometries or
coordinate reference systems (CRS) details. ⇒can’t store attribute
data or meta data
- Often used in database queries and as a lightweight geometry
exchange format.
- GML
- An XML-based format for encoding geospatial information. ⇒integrates
well with systems like SOAP services
- Object Vs. Field
- Geoid＋sphere/ellipsoid=datum
- Geoid is dynamic and ellipsoid is more theoretical, we can align both of
them
- If the alignment is local, where elliposid surface is closely fit to
the geoid at a particular location on earth’s services ⇒Local
datum(vertical)
- defines elevation (height) relative to a local reference
surface, often the mean sea level (MSL)
- Or Geocentric where the ellipsoid is aligned with the center of
the earth ⇒Geocentric datum(horizontal) E.g., WGS84
- defines the horizontal position (latitude and longitude) on
the Earth's surface based on a reference ellipsoid that is
centered at or near the Earth's center of mass.
- How we choose to align the ellipsoid to the geoid defines a datum
W1
Components of GIS
1. 2 (or 3) dimensional spatial component (here: horizontal plane)
2. An (often implicit) temporal component (here: z-axis)
3. A thematic component (here: symbology)
,Reference systems
1. Established sets of rules for the measurement of phenomena and their
relations
2. Constrains inter-subjective interpretation and reference to these phenomena
⇒meausrement symbols obtain an inter-subjective meaning and become
reproducible
3. Reference systems constrain operations one can apply to data
4. Adding up values requires the same reference system
Attribute reference systems
- Interpretations of signs into a domain of measurement ⇒E.g., interpretation of
“1” into a length(meter) ⇒fixed by convention (think about the prototype
meter)
- E.g., Meter scale for measuring lengths and distances
Temporal reference systems
- Can be linear (e.g., “unix time” seconds since 1 January 1970) or circular
(weekdays, months, hours of a day…)
- ISO 8601 data elements and interchange format, based on the Gregorian
calendar(Greenwich mean time)
- Forms the basis of xsd: dateTime
CRS(coordinate reference systems)
, - Datums: A datum is the information that fixes a coordinate system to some
reference object (the Earth, Geoid)
- Horizontal datums:
- Ellipsoid model of Earth with geodetic (geographic
coordinates)
- Map projection(projected coordinates)
- Vertical datums(fixing height). E.g., ellipsoidal height
Geodetic datums for geographic coordinates
- Horizontal(geodetic) datum consist of:
- Reference Elliposid(defined by major and minor axis lengths)
- …fixed relative to the earth via a standard position(fundamental point)
- …some standard orientation(of minor axis, aligned with poles)
- …and the location of prime meridian子午線(0 degree, e.g., Greenwich)
- Different “realizations” of datums in different years (e.g., WHS84=world
geodetic system of 1984)
Geographic datum shifts: positional errors
- Shifts of coordinate positions (in meters) between different realizations of the
North American Datum (NAD)
- Note: errors can be more than 80 meters in CA ⇒if you have a dataset based
on NAD 83 but you measure them as NAD 27, it will cause errors
Map projections
- How to get from an ellipsoid to the plane? Not without information loss!
- Three types of planar projections:
- Gnomic(from center)
- Stereographic(from opposite side)
- Orthographic(parallel projection)
- This projection is often used in mapping polar regions but can be used
for any location on the earth’s surface (in which case they are called
oblique planar
projections).
- Cylindrical projection
- A cylindrical map projection maps the earth surface onto a map rolled
into a cylinder (which can then be flattened into a plane). The cylinder
, can touch the surface of the earth along a single line of tangency (a
tangent case), or two lines of tangency(a secant case)
- Distortion is minimized along the tangent or secant lines and
increases as the distance from these lines increases
- E.g., UTM⇒Mercator, cylindrical equal-area
- Conical projection
- A conical map projection maps the earth surface onto a map rolled into
a cone. Like the cylindrical projection, the cone can touch the surface
of the earth along a single line of tangency (a tangent case),or two
lines of tangency(a secant case)
- E.g., Conical projections are also popular PCS’ in European maps such
as Europe Albers Equal Area Conic and Europe Lambert Conformal
Conic.
-
- Defined by
- class(cylindrical, conical or azimuthal)
- Point of secancy(tangent or secant)
- aspect(normal, transverse or oblique)
- Distortion property(equal area equidistant or conformal, or no
property)
,CRS transformations
- To homogenize geodata, involving 3 principal approaches:
- Geocentric transformations (via geocentric cartesian coordinates)
- Geographic transformations (via coordinate offsets)
, - 2D plane(affine,polynomial) transformations
Geodata access and retrieval
Interoperability
- Technical interoperability is the ability of different software systems to
communicate and interact via shared interfaces
- Technical interoperability for data and software can be solved via
- Standardized data formats
- Standardized web services
- Standardized interfaces (APIs)
- Semantic interoperability is the ability to search, find, and make use of data
from different sources (providers, organizations, measurements)
Basic geodata formats: raster and vector
- Raster encodes space into georeferenced cells
, - Vector encodes space into georeferenced points, lines and areas
- Important data formats:
- Vector: shp(shapefile,ESRI),WKT(well known text), GML(Geography
markup language), GeoJSON
- Raster: GeoTIFF, netCDF, IMG,GRID(ESRI)
Web service architecture and RESTful APIS
Typical software architectures
- Client-server
- 3-tier
- Presentation: user interface
- Application logic: calculations, decisions, transformations etc
- Data: storage and retrieval of data
-
Distribute tiers on the Web
- Web applications
- N-tier architecture: the presentation layer is a web page/site
- Many possible requests and server responses
- Many alternative implementations with varying requirements
What are Web API’s?
- API=application programming interface
- Exchange of data between a web service and a program
- Use of web services from within a program
How to access API ⇒http requests
- Http requests are fired against an endpoint)computing device communicating
back and forth)
-
,Restful services (representational state transfer)
- Uniform interface:
- Http GET request to receive data
- Http POST request to push data
- Stateless: no client is stored on the server between requests. Each request
from any client contains all the information necessary to service the request
- Individual resources are identified using URIs in Web-based REST systems
- Makes services easy to access via standard clients(URI and HTTP) as
opposed to SOAP(simple object access protocol) protocol
Geosocial media: foursquare and Open street map(OSM)
Problems
- Rate limits: the amount of data you can get is usually limited
- Read the social media platforms for details
- Low quantities are usually free, large quantities can still be obtained
with fees
- Frequency and amount of queries are restricted(denial of service)
- API changes: companies are free to change syntax or stop services
- Your code may need to be adapted once services are changed.
- Legal: restrictions in data usage for certain purposes
- Data quality: social media data is far from an unbiased sample
Linked open data(LOD) principles: link data on the web, not (only) documents
- Use URI to denote things
- Use HTTP URI so that these things can be referred to and looked up
(“dereferenced”) by people and user agents
- Provide useful information about the thing when its URL is dereferenced,
leveraging standards such as RDF, SPARQL
Open geographic information standards
- Open standards are a result of consensus delivered by international
organizations
- For geographic information
- ISO standards
- ISO 19115:2003 defines the schema required for describing
geographic information and services. It provides information
about the identification, the extent, the quality, the spatial and
temporal schema, spatial reference, and distribution of digital
geographic data.
- Open geospatial consortium(OGC) specifications
- For web standards
- W3C(html,RDF,XML,HTTP…)
- Spatial data on the Web Best practices
- Currently, most geodata services do not yet adhere to such RESTful/LOD
standards⇒still with SOAP standards
, OGC simple features(ISO19125)
- Geometry model where geometries are described by straight line segments or
planar interpolation between point sets (point, linestring, polygon)...
- Encoded as either Geography markup(GML) or Well-known text(WKT)
- WKT(Well known text)
- WKT is an OGC and ISO standard for representing geometries,
coordinate reference systems, and transformations
WKT vs GML vs Geojson
- WKT:
- A text-based format for representing vector geometry (points, lines,
polygons, etc.).
- Compact and human-readable (E.g., POINT(30 10) LINESTRING(30
10, 10…)
- No support for complex data structures like multi-geometries or
coordinate reference systems (CRS) details. ⇒can’t store attribute
data or meta data
- Often used in database queries and as a lightweight geometry
exchange format.
- GML
- An XML-based format for encoding geospatial information. ⇒integrates
well with systems like SOAP services
