has a great script to convert utm coordinates to lat/lng. However, I need to modify function below (taken from the script which can be download from above), to do mercator - lat/lng conversion.
Thanks for the help!!!

function UTMRefToLatLng() { var wgs84 = new RefEll(6378137, 6356752.314); var UTM_F0 = 0.9996; var a = wgs84.maj; var eSquared = wgs84.ecc; var ePrimeSquared = eSquared / (1.0 - eSquared); var e1 = (1 - Math.sqrt(1 - eSquared)) / (1 + Math.sqrt(1 - eSquared)); var x = this.easting - 500000.0;; var y = this.northing; var zoneNumber = this.lngZone; var zoneLetter = this.latZone; var longitudeOrigin = (zoneNumber - 1.0) * 6.0 - 180.0 + 3.0; // Correct y for southern hemisphere if ((ord(zoneLetter) - ord("N")) < 0) { y -= 10000000.0; } var m = y / UTM_F0; var mu = m / (a * (1.0 - eSquared / 4.0 - 3.0 * eSquared * eSquared / 64.0 - 5.0 * Math.pow(eSquared, 3.0) / 256.0)); var phi1Rad = mu + (3.0 * e1 / 2.0 - 27.0 * Math.pow(e1, 3.0) / 32.0) * Math.sin(2.0 * mu) + (21.0 * e1 * e1 / 16.0 - 55.0 * Math.pow(e1, 4.0) / 32.0) * Math.sin(4.0 * mu) + (151.0 * Math.pow(e1, 3.0) / 96.0) * Math.sin(6.0 * mu); var n = a / Math.sqrt(1.0 - eSquared * Math.sin(phi1Rad) * Math.sin(phi1Rad)); var t = Math.tan(phi1Rad) * Math.tan(phi1Rad); var c = ePrimeSquared * Math.cos(phi1Rad) * Math.cos(phi1Rad); var r = a * (1.0 - eSquared) / Math.pow( 1.0 - eSquared * Math.sin(phi1Rad) * Math.sin(phi1Rad), 1.5); var d = x / (n * UTM_F0); var latitude = ( phi1Rad - (n * Math.tan(phi1Rad) / r) * (d * d / 2.0 - (5.0 + (3.0 * t) + (10.0 * c) - (4.0 * c * c) - (9.0 * ePrimeSquared)) * Math.pow(d, 4.0) / 24.0 + (61.0 + (90.0 * t) + (298.0 * c) + (45.0 * t * t) - (252.0 * ePrimeSquared) - (3.0 * c * c)) * Math.pow(d, 6.0) / 720.0)) * (180.0 / Math.PI); var longitude = longitudeOrigin + ( (d - (1.0 + 2.0 * t + c) * Math.pow(d, 3.0) / 6.0 + (5.0 - (2.0 * c) + (28.0 * t) - (3.0 * c * c) + (8.0 * ePrimeSquared) + (24.0 * t * t)) * Math.pow(d, 5.0) / 120.0) / Math.cos(phi1Rad)) * (180.0 / Math.PI); return new LatLng(latitude, longitude);

however, a conversion from UTM - Mercator would also work if anyone has that!!

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tan^-1 is the inverse tangent - I think that's Math.atan in JavaScript.
sinh is the hyperbolic sine - I'm not sure if that exists in JavaScript, but if not, you can use this equality :

sinh(x) = (exp(x) - exp(-x)) / 2

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chrismarxAuthor Commented:

thanks, i will give this a try today and get back to you

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chrismarxAuthor Commented:

ok,
below is the javascript, but it does not seem to give the correct results. Im particularly curious about the forumla for lon -- how can lon be the mercator X coords + center lon? a typical mercator coordinate is
(x,y)
1851604, 331835

var sinh = (Math.exp(coordsY) - Math.exp(-coordsY)) / 2; var degrees = centerLatLng.substring(0,centerLatLng.indexOf("Â°"))/1; var minutes = centerLatLng.substring(centerLatLng.indexOf("Â°")+1,centerLatLng.indexOf("."))/ 60; var seconds = centerLatLng.substring(centerLatLng.indexOf("."),centerLatLng.indexOf("'")) / 3600; var centerLng = degrees + minutes + seconds; var lat = Math.atan(sinh); var lng = coordsX + centerLng;

>> how can lon be the mercator X coords + center lon?

You have to use the same units for the x,y and lat,lon values of course. I would prefer radians. (if they're not the same, you'll have to convert them).

That way, the center longitude of the Mercator map added to the Mercator x coordinate would then obviously give you the longitude.

>> 1851604, 331835

What's the unit ? That doesn't look like standard Mercator coordinates ...

Finally, also check what units the standard JavaScript functions expect ... Probably radians, but I'm not sure.

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chrismarxAuthor Commented:

ok, i'm a little rusty here, those coords are in meters. i have no idea how i would convert that to radians, do lat lng also convert to radians?

i think you're right about the radians things, Math.exp(coordsY) returns infinity....

>> ok, i'm a little rusty here, those coords are in meters.

That's not the standard Mercator projection afaik.

Luckily it's easy to convert meters (m) to radians (rad) :

rad = m / R

with R the radius of the Earth (average here) :

R = 6372795.477598 m

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chrismarxAuthor Commented:

ok! great, see the script below. Lng is now returning the correct coordinates, but lat is still way off, perhaps you can spot the problem. sinh is returning numbers like 0.0345079, the Math.atan() doesn't seem to change this number either-

var rad = 6372795.477598;var sinh = (Math.exp(coordsY/rad) - Math.exp(-coordsY/rad)) / 2;alert(sinh);var lat = Math.atan(sinh);var degrees = centerLatLng.substring(0,centerLatLng.indexOf("Â°"))/1;var minutes = centerLatLng.substring(centerLatLng.indexOf("Â°")+1,centerLatLng.indexOf("."))/ 60; var seconds = centerLatLng.substring(centerLatLng.indexOf("."),centerLatLng.indexOf("'")) / 3600;var centerLng = degrees + minutes + seconds;var lng = (coordsX/rad) + (centerLng);

hmm.... yeah, something is still not working here, i really appreciate your patience. here's all the info:

here are the coords of the center of my map in mercator (this is western ny on my map)
1143031.34, 835295.19

the center longitude is 81.16691694444445 (and the correct center lat should be 42. etc)

the new code is below. none of these values correspond to what im expecting. when i reproject the map im using to utm, and use the above script, i get the correct lat/lng, so i think the mercator coords are correct.

var rad = 6372795.477598;var sinh = (Math.exp(coordsY/rad) - Math.exp(-coordsY/rad)) / 2;var lat = Math.atan(sinh)*(180/Math.PI);var lngrad = (coordsX/rad) + ((centerLng)*(Math.PI/180))var lng = (lngrad)*(180/Math.PI);

>> here are the coords of the center of my map in mercator

??? The center of the map should be the (0,0) point, and it should have a corresponding (lat,lon) pair, usually, the equator and the 0-meridian.

>> the center longitude is 81.16691694444445 (and the correct center lat should be 42. etc)

Are you sure this is a Mercator map ?

>> when i reproject the map im using to utm

Which is it now ... is it UTM as I suspected earlier ? Or is it a basic Mercator projection ?

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chrismarxAuthor Commented:

oh, i thought you meant the center of my current map, which, being in western ny, are those coordinates.

yes, i am working with a gis internet mapping application, and it returns enough information for me to calculate the correct projected coodinates; it also returns the center lat/lng (but nothing more). In the GIS, the projection is indeed mercator. When i reproject the map to use UTM, the coords are very different. are there different versions of mercator? is there any way to check whether the coords im using represent what you think of as mercator?

A Mercator projection is just a type of projection. The original one is sometimes still used for maps

>> is there any way to check whether the coords im using represent what you think of as mercator?

Is this a world map ? If not, what does the map contain ? What are the maximum and minimum (Mercator) coordinates ? What's the center latitude/longitude ?

Can you also give a known correct pair of Mercator coordinates and the corresponding latitude/longitude ?

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chrismarxAuthor Commented:

ok, below is the projection/datum info. i have a series of maps, but they're all generally around ohio/western ny.

mercator
World Geodetic 1984 (WGS84) Auto
Center Latitude 0.00
Center Longitude 0.00

Local offset -9367812.99, 4954053.105
local scale 0.298582119, .29858213
scale correction 1.00, 1.00, 1.00
false easting/northing 0.00, 0.00

here are some coordinate pairs
1059121.38, 855806.52 || -81.3117, 42.3236
1661408.43, 1124318.39 || -79.6963, 42.8539

This is the offset to the center of the map, and with the earlier formulae, you get this corresponding latitude, longitude pair :

lat0 = atan(sinh(4954053.105/R)) = 0.70916 rad = 40.63195Â°
lon0 = -9367812.99/R + 0.0 = -1.46997 rad = -84.22303Â°

So, that's our center.

Now the coordinate pair examples you mentioned :

>> 1059121.38, 855806.52 || -81.3117, 42.3236

lat = atan(sinh(855806.52/R)) + 0.70916 = 0.84305 rad = 48.30321Â°
lon = 1059121.38/R - 1.46997 = -1.30378 rad = -74.70085Â°

>> 1661408.43, 1124318.39 || -79.6963, 42.8539

lat = atan(sinh(1124318.39/R)) + 0.70916 = 0.88468 rad = 50.68823Â°
lon = 1661408.43/R - 1.46997 = -1.20927 rad = -69.28588Â°

As you see, they're not a perfect match, so it's probably not standard Mercator. This seems to confirm it :

>> World Geodetic 1984 (WGS84) Auto

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chrismarxAuthor Commented:

ok, yes, i've dealt with this before. I just need to compensate for the local scale and offset. the code below does that. now (finally - thank you!!), im am getting the correct latitudes given the mercator coordinates. longitude is still off, for this region, im getting around -165. the code is below

var rad = 6372795.477598;var centerLng = (-9367812.99732577/rad); var X = ((coordsX*0.2985821196282)-9367812.99732577);var Y = coordsY*0.2985821374044+(4954053.105471344);var sinh = (Math.exp(Y/rad) - Math.exp(-Y/rad)) / 2;var lat = Math.atan(sinh)*(180/Math.PI);var lngrad = (X/rad) + centerLng;var lng = (lngrad)*(180/Math.PI);

oh, my bad, i was using the center you specified, not 0.0
now it works, thank you!!!

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chrismarxAuthor Commented:

ok, i can open another question for this one if you want, but heres definitely the last question. the forumla is working now, but the lat/lng coords are still off by around .4 degrees. any way to make this more accurate?

I'm not sure, but I would suspect that that's due to rounding errors, either in the input data, or in the calculations. Or maybe the verification values are a bit off ?

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chrismarxAuthor Commented:

oh i found the problem. i was using your ellipsoid value, but i should have been using the ellipsoid for wgs84

There are many ways to learn to code these days. From coding bootcamps like Flatiron School to online courses to totally free beginner resources. The best way to learn to code depends on many factors, but the most important one is you. See what course is best for you.