How can we define Latitude and Longitude before defining a Datum?

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Reference material:



The Five Key Ingredients for Mapping the Earth



There are five important steps to producing a map of the Earth's Surface. These are:
sphere with north and south through the middle:



Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.



For maps of large areas of the Earth this would be latitude/longitude and for simpler ‘sketches’ or maps of local areas this would be a grid. Some more complex maps may use both latitude/longitude and a grid reference system.
Spheroid Diagram



Secondly, a Datum which defines the geometry/mathematics of how to transfer this coordinate system onto the surface of the Earth.



This is necessary for all maps of large areas of the Earth, but may not be needed for simpler ‘sketches’ or maps of local areas where features are able to be visually located relative to each other.



A feature of the modern electronic era is that many map users are now also using Global Positioning Systems (GPS) when they are using maps. An interesting impact of this is that more and more maps are being forced to use the same datum as the GPS system (even ‘sketches’ and maps of local areas maps) so that the reference coordinates on the map agree with the coordinates shown on the GPS receiver.
Surveyor using surveying equipment.



Thirdly, a measurement system which is used calculate the coordinates of points on the surface of the Earth.
World map.



Fourthly, a projection which allows the coordinates, which have been calculated using a measuring system, to be displayed on a flat piece of paper.
Colourful timezone map of Australasia.



Fifthly, the Map Maker’s Art.



Link: https://www.icsm.gov.au/education/fundamentals-mapping/overview-fundamentals-mapping



Question:
My perception of Latitude and Longitude is that:



Since, earth is wobbly in shape, we define a datum with a particular center and particular shape. Using that datum as a reference, write down latitude and longitude into that framework.



But according to the given link and the snippet of information presented, latitude and longitude are defined first and then a datum is created to transfer the coordinate system onto the surface of the earth.



The confusion now is then, on what basis/surface/reference the latitude and longitude are defined upon?






latitude-longitude datum mapping







share|improve this question



























    up vote
    2
    down vote

    favorite












    Reference material:



    The Five Key Ingredients for Mapping the Earth



    There are five important steps to producing a map of the Earth's Surface. These are:
    sphere with north and south through the middle:



    Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.



    For maps of large areas of the Earth this would be latitude/longitude and for simpler ‘sketches’ or maps of local areas this would be a grid. Some more complex maps may use both latitude/longitude and a grid reference system.
    Spheroid Diagram



    Secondly, a Datum which defines the geometry/mathematics of how to transfer this coordinate system onto the surface of the Earth.



    This is necessary for all maps of large areas of the Earth, but may not be needed for simpler ‘sketches’ or maps of local areas where features are able to be visually located relative to each other.



    A feature of the modern electronic era is that many map users are now also using Global Positioning Systems (GPS) when they are using maps. An interesting impact of this is that more and more maps are being forced to use the same datum as the GPS system (even ‘sketches’ and maps of local areas maps) so that the reference coordinates on the map agree with the coordinates shown on the GPS receiver.
    Surveyor using surveying equipment.



    Thirdly, a measurement system which is used calculate the coordinates of points on the surface of the Earth.
    World map.



    Fourthly, a projection which allows the coordinates, which have been calculated using a measuring system, to be displayed on a flat piece of paper.
    Colourful timezone map of Australasia.



    Fifthly, the Map Maker’s Art.



    Link: https://www.icsm.gov.au/education/fundamentals-mapping/overview-fundamentals-mapping



    Question:
    My perception of Latitude and Longitude is that:



    Since, earth is wobbly in shape, we define a datum with a particular center and particular shape. Using that datum as a reference, write down latitude and longitude into that framework.



    But according to the given link and the snippet of information presented, latitude and longitude are defined first and then a datum is created to transfer the coordinate system onto the surface of the earth.



    The confusion now is then, on what basis/surface/reference the latitude and longitude are defined upon?






    latitude-longitude datum mapping







    share|improve this question























      up vote
      2
      down vote

      favorite









      up vote
      2
      down vote

      favorite











      Reference material:



      The Five Key Ingredients for Mapping the Earth



      There are five important steps to producing a map of the Earth's Surface. These are:
      sphere with north and south through the middle:



      Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.



      For maps of large areas of the Earth this would be latitude/longitude and for simpler ‘sketches’ or maps of local areas this would be a grid. Some more complex maps may use both latitude/longitude and a grid reference system.
      Spheroid Diagram



      Secondly, a Datum which defines the geometry/mathematics of how to transfer this coordinate system onto the surface of the Earth.



      This is necessary for all maps of large areas of the Earth, but may not be needed for simpler ‘sketches’ or maps of local areas where features are able to be visually located relative to each other.



      A feature of the modern electronic era is that many map users are now also using Global Positioning Systems (GPS) when they are using maps. An interesting impact of this is that more and more maps are being forced to use the same datum as the GPS system (even ‘sketches’ and maps of local areas maps) so that the reference coordinates on the map agree with the coordinates shown on the GPS receiver.
      Surveyor using surveying equipment.



      Thirdly, a measurement system which is used calculate the coordinates of points on the surface of the Earth.
      World map.



      Fourthly, a projection which allows the coordinates, which have been calculated using a measuring system, to be displayed on a flat piece of paper.
      Colourful timezone map of Australasia.



      Fifthly, the Map Maker’s Art.



      Link: https://www.icsm.gov.au/education/fundamentals-mapping/overview-fundamentals-mapping



      Question:
      My perception of Latitude and Longitude is that:



      Since, earth is wobbly in shape, we define a datum with a particular center and particular shape. Using that datum as a reference, write down latitude and longitude into that framework.



      But according to the given link and the snippet of information presented, latitude and longitude are defined first and then a datum is created to transfer the coordinate system onto the surface of the earth.



      The confusion now is then, on what basis/surface/reference the latitude and longitude are defined upon?






      latitude-longitude datum mapping







      share|improve this question













      Reference material:



      The Five Key Ingredients for Mapping the Earth



      There are five important steps to producing a map of the Earth's Surface. These are:
      sphere with north and south through the middle:



      Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.



      For maps of large areas of the Earth this would be latitude/longitude and for simpler ‘sketches’ or maps of local areas this would be a grid. Some more complex maps may use both latitude/longitude and a grid reference system.
      Spheroid Diagram



      Secondly, a Datum which defines the geometry/mathematics of how to transfer this coordinate system onto the surface of the Earth.



      This is necessary for all maps of large areas of the Earth, but may not be needed for simpler ‘sketches’ or maps of local areas where features are able to be visually located relative to each other.



      A feature of the modern electronic era is that many map users are now also using Global Positioning Systems (GPS) when they are using maps. An interesting impact of this is that more and more maps are being forced to use the same datum as the GPS system (even ‘sketches’ and maps of local areas maps) so that the reference coordinates on the map agree with the coordinates shown on the GPS receiver.
      Surveyor using surveying equipment.



      Thirdly, a measurement system which is used calculate the coordinates of points on the surface of the Earth.
      World map.



      Fourthly, a projection which allows the coordinates, which have been calculated using a measuring system, to be displayed on a flat piece of paper.
      Colourful timezone map of Australasia.



      Fifthly, the Map Maker’s Art.



      Link: https://www.icsm.gov.au/education/fundamentals-mapping/overview-fundamentals-mapping



      Question:
      My perception of Latitude and Longitude is that:



      Since, earth is wobbly in shape, we define a datum with a particular center and particular shape. Using that datum as a reference, write down latitude and longitude into that framework.



      But according to the given link and the snippet of information presented, latitude and longitude are defined first and then a datum is created to transfer the coordinate system onto the surface of the earth.



      The confusion now is then, on what basis/surface/reference the latitude and longitude are defined upon?






      latitude-longitude datum mapping




      latitude-longitude datum mapping






      share|improve this question













      share|improve this question











      share|improve this question




      share|improve this question










      asked 1 hour ago









      Tarun Maganti

      1137




      1137




















          2 Answers
          2






          active

          oldest

          votes

















          up vote
          2
          down vote



          accepted










          Latitude and Longitude are defined upon an ellipsoid.




          Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.




          (The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.)
          So back to your question and numbering of thigs:



          1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).


          2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.






          share|improve this answer






















          • So, the first point is basically defining an ellipsoid?
            – Tarun Maganti
            10 mins ago










          • If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
            – Tarun Maganti
            8 mins ago

















          up vote
          1
          down vote













          In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))






          share|improve this answer




















          • Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
            – Tarun Maganti
            11 mins ago










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          2 Answers
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          active

          oldest

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          2 Answers
          2






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          2
          down vote



          accepted










          Latitude and Longitude are defined upon an ellipsoid.




          Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.




          (The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.)
          So back to your question and numbering of thigs:



          1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).


          2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.






          share|improve this answer






















          • So, the first point is basically defining an ellipsoid?
            – Tarun Maganti
            10 mins ago










          • If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
            – Tarun Maganti
            8 mins ago














          up vote
          2
          down vote



          accepted










          Latitude and Longitude are defined upon an ellipsoid.




          Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.




          (The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.)
          So back to your question and numbering of thigs:



          1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).


          2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.






          share|improve this answer






















          • So, the first point is basically defining an ellipsoid?
            – Tarun Maganti
            10 mins ago










          • If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
            – Tarun Maganti
            8 mins ago












          up vote
          2
          down vote



          accepted







          up vote
          2
          down vote



          accepted






          Latitude and Longitude are defined upon an ellipsoid.




          Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.




          (The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.)
          So back to your question and numbering of thigs:



          1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).


          2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.






          share|improve this answer














          Latitude and Longitude are defined upon an ellipsoid.




          Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.




          (The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.)
          So back to your question and numbering of thigs:



          1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).


          2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited 1 hour ago

























          answered 1 hour ago









          Roberto Mallorca

          1265




          1265











          • So, the first point is basically defining an ellipsoid?
            – Tarun Maganti
            10 mins ago










          • If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
            – Tarun Maganti
            8 mins ago
















          • So, the first point is basically defining an ellipsoid?
            – Tarun Maganti
            10 mins ago










          • If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
            – Tarun Maganti
            8 mins ago















          So, the first point is basically defining an ellipsoid?
          – Tarun Maganti
          10 mins ago




          So, the first point is basically defining an ellipsoid?
          – Tarun Maganti
          10 mins ago












          If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
          – Tarun Maganti
          8 mins ago




          If so, isn't this: Firstly, a reference system which is used to describe the locations of points on the surface of the Earth. also datum? Because you are locating or pointing the points on the surface of the earth.
          – Tarun Maganti
          8 mins ago












          up vote
          1
          down vote













          In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))






          share|improve this answer




















          • Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
            – Tarun Maganti
            11 mins ago














          up vote
          1
          down vote













          In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))






          share|improve this answer




















          • Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
            – Tarun Maganti
            11 mins ago












          up vote
          1
          down vote










          up vote
          1
          down vote









          In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))






          share|improve this answer












          In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered 1 hour ago









          radouxju

          38.4k136109




          38.4k136109











          • Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
            – Tarun Maganti
            11 mins ago
















          • Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
            – Tarun Maganti
            11 mins ago















          Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
          – Tarun Maganti
          11 mins ago




          Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area?
          – Tarun Maganti
          11 mins ago

















           

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