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I am a software developer and I am building an app that involves working with Longitude and Latitude. I've been surfing the web to understand the subject well but still have some issues with it that I hope you give me a hand to understand it right.

I know that the size/width of longitude is about 54.6 miles. It could increase/decrease when you move up or down to North or South.

I have three questions, and I used the following picture to help me convey the information to you...

help picture

  1. As long as the man travels south, north, east or west but he's still in the blue area of that specific longitude which is of i.e 31 degree, the longitude would still be the same(31), right?
  2. When he gets out of the blue area and passes it's barriers through going east/west, the longitude should change to add decimal places to the main degree(31) i.e 31.1234 or maybe 30.99999 instead of 31 if he passes the barriers and enters another longitude of degree 30.
  3. The same goes for Latitudes but in the contrary?

Am I understanding it correctly?

J. Monticolo
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Eslam
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    Not really. You're probably thinking about graticules with lines at every whole degree. Longitude and latitude are angles in degrees and are continuous. Take a step to the east or west and your longitude has changed, even if only very slightly. – user2856 Jun 14 '23 at 02:52
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    You need to spend some time with with Wikipedia page on the Problems of Geodesy. The actual distance between constant steps in lat/lon values is a partial differential equation based on the change in latitude. The distance for one degree of change varies between sixty nautical miles at the Equator and zero at the poles. – Vince Jun 14 '23 at 03:42
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    If you're talking about distance calculations you will need to research projected coordinate systems, Google Web Mercator https://epsg.io/3857 is ok if you don't need to be super accurate or from a given lat/lon you can determine which UTM zone https://www.usgs.gov/faqs/how-are-utm-coordinates-measured-usgs-topographic-maps#:~:text=If%20UTM%20ticks%20are%20shown,equator%20in%20a%20northerly%20direction. is appropriate and then project to that. I advise not to do the hard work yourself and use GDAL/OSR https://gdal.org/java/org/gdal/osr/SpatialReference.html or similar to do the grunt work. – Michael Stimson Jun 14 '23 at 03:54
  • Install Google Earth (free) to get an intuitive feeling what happens when you move on Earth as you see coordinate values and can measure distances on different zoom levels. Longitude has no "size", it is a measurement/coordinate value that indicates a position on the globe and of course, every movement (theoretically even moving just one inch east or west) changes the longitude value, even in a small amount. – Babel Jun 14 '23 at 07:05
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    @MichaelStimson Web Mercator is certainly *not* "ok" for distance measurement, except along the Equator. Using the Haversine formula is "ok if you don't need to be super accurate" (since it works on a sphere). Using an equidistant projection is a pain, because it's only good for one location. A local projection is a good solution, provided your distances are within the useful map extent. Using a geodetic function works for all distances short of antipodal. – Vince Jun 14 '23 at 11:28

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