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task #7917

Revise ReferenceSystemTerms

Added by Andreas Kohlbecker 2 months ago. Updated about 2 months ago.

Status:
Feedback
Priority:
New
Category:
cdmlib
Target version:
Start date:
11/20/2018
Due date:
% Done:

30%

Severity:
normal
Tags:

Description

A spatial reference system (SRS) or coordinate reference system (CRS) is a coordinate-based local, regional or global system used to locate geographical entities.
Some of the default terms provided by the eu.etaxonomy.cdm.model.location.ReferenceSystem class, do not conform to a reference system:

  • WGS84: Is the World Geodetic System 84 also known as WGS 1984, EPSG:4326, this one is perfectly ok
  • GoogleEarth: makes no sense if names like this. GoogleEarth is the name of an application. This term should be renamed to one of Web Mercator, Google Web Mercator, Spherical Mercator, and has the code is EPSG:900913
  • Gazetteer: is a geographical dictionary or directory used in conjunction with a map or atlas, it is not a reference system and must be removed
  • Map: makes no sense at all as reference system, remove it!

Background information on geodetic reference systems:

A geodetic datum or geodetic system is a coordinate system, and a set of reference points, used to locate places on the Earth (or similar objects). Reference systems are composed of a couple of constructs like an ellipsoid, geoid, anchor coordinates, etc.

ellipsoid

In geodetic coordinates, the Earth's surface is approximated by an ellipsoid. An Earth ellipsoid is a mathematical figure approximating the Earth's form, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different ellipsoids have been used as approximations.


(http://www.kowoma.de/gps/geo/mapdatum.htm)

"Auch wenn zwei Länder dasselbe Ellipsoid verwenden (z. B. Deutschland und Österreich das Bessel-Ellipsoid), unterscheiden sie sich doch in diesem Zentralpunkt bzw. Fundamentalpunkt. Daher können sich die Koordinaten der gemeinsamen Grenzpunkte um bis zu einem Kilometer unterscheiden. ... Das Geoid gibt also sozusagen Normal-Null für die gesamten Erde an. So definiert das WGS-84 Geoid Höhen für jeden Punkt auf der gesamten Erde." (https://de.wikipedia.org/wiki/Referenzellipsoid)

geoid

The geoid is the shape that the ocean surface would take under the influence of the gravity and rotation of Earth alone, if other influences such as winds and tides were absent. This surface is extended through the continents (such as with very narrow hypothetical canals).

"Die Meereshöhe ist die durchschnittliche Höhe der Ozeane. Gezeitenkräfte und lokale Gravitationsunterschiede bewirken weltweite Unterschiede dieser Höhe im Bereich von Hunderten von Metern. Wäre überall auf der Erde nur Wasser, so hätte aufgrund der Gravitationsunterschiede die Erde trotzallem weder die Form einer Kugel noch die eines einfachen Ellipsoiden." (http://www.kowoma.de/gps/geo/mapdatum.htm)

For us it is important to note that a geodetic reference systems is an ideal approximation to the shape and form of the earth. Any coordinate (longitude / latitude) defines a point in this geometric structure. A coordinate in a geodetic system can not be compared to the coordinate of another system without having the knowledge on the reference systems.

Ein falsches Kartendatum kann zu Fehlern bei der Positionsbestimmung von mehreren hundert Metern führen.

Links:

Conclusions

  1. Without the the geodetic reference system logitute and latitute are in accurate and can differ from the actual location by up to several 100m.
  2. ReferenceSystemTerms should preferrably have a link to the according entry in http://spatialreference.org so that automatic calulations can be done.
  3. The terms Gazetteer, Map are measurement methods and must not be mixed with geodetic reference system
  4. The term GoogleEarth should be translated into the reference system used by google (WGS 84): https://gis.stackexchange.com/questions/20259/what-datum-reference-ellipsoid-does-google-earth-use#20265 --> "The heights on google earth refer to EGM96 and are, therefore, Geoidal heights. The lat/long are referred to the WGS 84 ellipsoid."

Related issues

Related to Edit - feature request #7218: SpecimenTypedesignationsWorkingsetEditor: support for Reference system Closed 01/25/2018

History

#1 Updated by Andreas Kohlbecker 2 months ago

  • Status changed from New to Feedback
  • Assignee changed from Andreas Kohlbecker to Andreas Müller
  • % Done changed from 0 to 20

Hi Andreas, please commment

#3 Updated by Andreas Kohlbecker about 2 months ago

I discovered that adding 'gazetteer' was an explicit requirement which was handled in #2374. What was the purpose, use case for adding this term?

#4 Updated by Andreas Kohlbecker about 2 months ago

  • Related to feature request #7218: SpecimenTypedesignationsWorkingsetEditor: support for Reference system added

#5 Updated by Wolf-Henning Kusber about 2 months ago

The entries are mixing Reference class and the method for finding the correct value. The method might be of interest, but only as the method.

#6 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)
  • % Done changed from 20 to 30

I did some further investigation on reference systems and their importance on geolocations. I adding these informations to the issue description.

The key insight is "A coordinate in a geodetic system can not be compared to the coordinate of another system without having the knowledge on the reference systems."

#7 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)

#8 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)

#9 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)

#10 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)

#11 Updated by Andreas Kohlbecker about 2 months ago

  • Description updated (diff)

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