Tudor Pavel

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Requirement:

Suggest parking areas close touser's location.

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Calculating distances betweenpoints ain't that easy.

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Of course, for small cities...

we can ignore the fact the planet is sphericaland see it as flat.

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But that's still a problem unless weproperly convert from degrees to

meters/kilometers.

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So we used this table...

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And this table...

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To implement a distance converter module

module DistanceConverter # ...

# Convert Latitude distance from Kilometers to Degrees def to_deg_lat(km:, lat: 45) km / ONE_LAT_DEGREE[deg_approx_lat(lat)] end

# Convert Latitude distance from Degrees to Kilometers def to_km_lat(deg:, lat: 45) deg * ONE_LAT_DEGREE[deg_approx_lat(lat)] end

# Convert Longitude distance from Kilometers to Degrees def to_deg_lng(km:, lat: 45) km / ONE_LNG_DEGREE[deg_approx_lat(lat)] end

# Convert Longitude distance from Degrees to Kilometers def to_km_lng(deg:, lat: 45) deg * ONE_LNG_DEGREE[deg_approx_lat(lat)] end

private

# ...end

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All is well, except...

This converter is Ruby. What if we need tocalculate distances in an SQL query?

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A naive versionSELECT (sqrt((43.1234 - latitude)̂2 + (23.1234 - longitude)̂2)) AS distFROM areas;

That's just the linear distance between two points in a plane, so it's correctwith our assumtion when in a small city?But the problem is it is using degrees which is a bigger problem the closerwe are to the poles.One longitude degree will be worth less kilometers than a latitude degree.So, what now?

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"Here I come to save the day!"PostGIS

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This is a driving distance 5.7 km.

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PostGIS version of the querySELECT ST_Distance_Sphere( ST_MakePoint(43.1234, 23.1234), ST_MakePoint(latitude, longitude)) AS distFROM areas;

For the example, the straight line distance with our Ruby converter: 4.5 kmPostGIS result: 6.5 kmWat

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Mister StackOverflow, please help

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After some research involving...

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Being reminded about projectiondistortion

https://gmaps-samples.googlecode.com/svn/trunk/poly/puzzledrag.html

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Even starting to doubt the Earth isactually a spheroid.

Why would they have more coordinatesystems??

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After a while...

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I looked closer at the docs

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Did you miss it?

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ConclusionsRead the docs.

Think about the simplest possibilities first.

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