3 ms·

Cool topic, but that article was a bit long and meandering. Here's a good supplement: https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/ https://gisge

by jzl 7y ago

Cool topic, but that article was a bit long and meandering. Here's a good supplement: https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/ https://gisgeography.com/geodetic-datums-nad27-nad83-wgs84/ Global coordinates in general are a lot more complicated than many people realize. For example, how exactly is GPS "elevation" determined? If you're standing on a beach at mean tide, is the elevation from a GPS receiver guaranteed to be zero everywhere on the planet? Here's a good starter on that one: https://eos-gnss.com/elevation-for-beginners https://eos-gnss.com/elevation-for-beginners

onion2k 7y ago
If you're standing on a beach at mean tide... What's "mean tide"? Wouldn't it be different around the world depending on the position of the moon (and the Sun..) relative to your location? The lowest tide (neap tide) is when the effect of the Sun and the moon partially cancel one another out, and the highest tide (spring tide) is when the Sun and Moon line up and add their effects on the tidal range. Sea level sucks as a baseline.
- globular-toast 7y ago
  Spring tides have both the highest and lowest tides. Neap tide is where the difference between high and low tide is the smallest.
  - fyfy18 7y ago
    So is the neap tide level zero elevation?
    - fhars 7y ago
      No, neap tide is the tide where the difference between low and high tide level is smallest.
- fhars 7y ago
  That's why you use the mean tide which averages over all the possible positions of the sun and the moon, not the momentary tide which, like many computer problems, depends on the time of the day and the phase of the moon.
- wffurr 7y ago
  MLLW is mean low low water. It's the average of the lowest tide each day at that location. That's the standard used on nautical charts and also used for tide charts for the depth of the tides each day.
  - znep 7y ago
    Yes, in the US (and presumably some other countries, I don't know of any offhand). Many other countries such as Canada use Lowest Astronomical Tide. The difference is actually important to know at times when boating between the countries. See https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_phase https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_ph...
- ringshall 7y ago
  I don't know why this was downvoted. Sea level does indeed suck as a baseline. Traditional vertical datums like NAVD88 get their baseline zero elevation from an averaging of mean sea level at a small number (~5) of tidal stations around the area of interest. The reason why multiple stations are used is that mean sea level is different from place to place. This is because of ocean currents and (I think though I can't say for certain) different levels of gravity at different points on earth's surface. This is most clearly demonstrated with chart datum, which is a local elevation datum particular to a single maritime chart (eg a single harbor). Chart datum is the mean low tide for the area in question (useful to know if you don't want your craft to run aground...). The point of mentioning this is that: there are different chart datums along a single seaboard. Mean sea level is local. The current practice is to use geoids which are based on the force of gravity. Einstein described geoids as surfaces on which clocks run at a constant rate. This is analogous to mean sea level on a waterworld in free space, eg sea level without the confounding factors of currents, weather, climate, or astronomical objects.
fhars 7y ago
Here is another nice anecdote about sea level not being sea level: https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600 https://www.science20.com/news_articles/what_happens_bridge_... (The article doesn't actually mention what happend, according to german wikipedia, the error was spotted early and the moderate costs for the correction efforts were covered by the architects' insurance policy.)
Merrill 7y ago
ESRI is a major US company in the Geographic Information Systems, mainly for government and business. >Coordinate systems, map projections, and geographic (datum) transformations >Every dataset has a coordinate system, which is used to integrate it with other geographic datasets within a common coordinate framework. Coordinate systems enable you to integrate datasets to perform various integrated analytical operations, such as overlaying data layers from disparate sources and coordinate systems. Geographic, projected, and vertical coordinate systems, as well as geographic (datum) transformations, are discussed in this topic. http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b720a5-7339-44b0-8b58-0f5bf2843393.htm http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b72... >US datum modernization coming in 2022 https://community.esri.com/groups/coordinate-reference-systems/blog/2019/02/26/us-datum-modernization-coming-in-2022 https://community.esri.com/groups/coordinate-reference-syste...

by jzl 7y ago

onion2k 7y ago

If you're standing on a beach at mean tide... What's "mean tide"? Wouldn't it be different around the world depending on the position of the moon (and the Sun..) relative to your location? The lowest tide (neap tide) is when the effect of the Sun and the moon partially cancel one another out, and the highest tide (spring tide) is when the Sun and Moon line up and add their effects on the tidal range. Sea level sucks as a baseline.

globular-toast 7y ago
Spring tides have both the highest and lowest tides. Neap tide is where the difference between high and low tide is the smallest.
- fyfy18 7y ago
  So is the neap tide level zero elevation?
  - fhars 7y ago
    No, neap tide is the tide where the difference between low and high tide level is smallest.
fhars 7y ago
That's why you use the mean tide which averages over all the possible positions of the sun and the moon, not the momentary tide which, like many computer problems, depends on the time of the day and the phase of the moon.
wffurr 7y ago
MLLW is mean low low water. It's the average of the lowest tide each day at that location. That's the standard used on nautical charts and also used for tide charts for the depth of the tides each day.
- znep 7y ago
  Yes, in the US (and presumably some other countries, I don't know of any offhand). Many other countries such as Canada use Lowest Astronomical Tide. The difference is actually important to know at times when boating between the countries. See https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_phase https://en.wikipedia.org/wiki/Chart_datum#Choice_of_tidal_ph...
ringshall 7y ago
I don't know why this was downvoted. Sea level does indeed suck as a baseline. Traditional vertical datums like NAVD88 get their baseline zero elevation from an averaging of mean sea level at a small number (~5) of tidal stations around the area of interest. The reason why multiple stations are used is that mean sea level is different from place to place. This is because of ocean currents and (I think though I can't say for certain) different levels of gravity at different points on earth's surface. This is most clearly demonstrated with chart datum, which is a local elevation datum particular to a single maritime chart (eg a single harbor). Chart datum is the mean low tide for the area in question (useful to know if you don't want your craft to run aground...). The point of mentioning this is that: there are different chart datums along a single seaboard. Mean sea level is local. The current practice is to use geoids which are based on the force of gravity. Einstein described geoids as surfaces on which clocks run at a constant rate. This is analogous to mean sea level on a waterworld in free space, eg sea level without the confounding factors of currents, weather, climate, or astronomical objects.

fhars 7y ago

Here is another nice anecdote about sea level not being sea level: https://www.science20.com/news_articles/what_happens_bridge_when_one_side_uses_mediterranean_sea_level_and_another_north_sea-121600 https://www.science20.com/news_articles/what_happens_bridge_... (The article doesn't actually mention what happend, according to german wikipedia, the error was spotted early and the moderate costs for the correction efforts were covered by the architects' insurance policy.)

Merrill 7y ago

ESRI is a major US company in the Geographic Information Systems, mainly for government and business. >Coordinate systems, map projections, and geographic (datum) transformations >Every dataset has a coordinate system, which is used to integrate it with other geographic datasets within a common coordinate framework. Coordinate systems enable you to integrate datasets to perform various integrated analytical operations, such as overlaying data layers from disparate sources and coordinate systems. Geographic, projected, and vertical coordinate systems, as well as geographic (datum) transformations, are discussed in this topic. http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b720a5-7339-44b0-8b58-0f5bf2843393.htm http://resources.esri.com/help/9.3/arcgisengine/dotnet/89b72... >US datum modernization coming in 2022 https://community.esri.com/groups/coordinate-reference-systems/blog/2019/02/26/us-datum-modernization-coming-in-2022 https://community.esri.com/groups/coordinate-reference-syste...