# How to differentiate coordinate systems by looking at coordinates

I just spent forever finding a DD MM SS to dd.dddddd converter so I can map/geocode/whatever this spreadsheet with oldskool lat and long. (Not my core job duty, but I have no trouble creating KMLs with V/H decimal coordinates).

I’m overwhelmed trying to figure out if these are NAD27/83/UTM/ or other to use this converter from US Army Corps of Engineers and can’t find a way to pinpoint the coordinate system without spending hours learning the history of cartography. The simpler converter I downloaded was jank and junk all in one.

Any help would be so much appreciated. Just in case, the lat/long looks like this (has neg/pos and some [2?] dec places but no cardinal NESW):

40 44 36 -84 05 52
40-44-29.96 -84 07 06.19
40 46 40.98 -84 04 09.55
40 42 07 -84 06 25
40 42 58.5 -84 04 36.9
40 45 24 -84 07 47
40 43 44.5 -84 08 33.1
40 43 43 -84 11 03
40 49 06 -84 00 12
40 50 58 -84 04 55
40 46 41.89 -84 10 42.31

As far as I know you cannot infer a datum from a co-ordinate set. A set of DD MM SS.SSS co-ords will look identical be they NAD27, WGS84, etc. They are not UTM which are a different co-ordinate format.

Cardinal NSEW aren’t needed for this set. +ve on the first set is North of the equator. -ve on the second set is West of 0 degrees lat (Prime Meridian).
Converting these to decimal degrees is pretty easy.

Keep the sign.
The first two are the degrees.
The second two are the minutes (so divide by 60).
The last are seconds (so divide by 3600).
Add the new decimal minutes and seconds together.
Append the sign and degrees and add a decimal.

40 44 36
40
44/60 = 0.733333333
36/3600 = 0.01
0.733333333 + 0.01 = 0.743333333
Result:
40.743333333

-84 05 52
-84
05/60 = 0.083333333
52/3600 = 0.014444444
0.083333333 + 0.014444444 = 0.097777778
Result:
-84.097777778

Final conversion is 40.743333333 -84.097777778

Plot them both in Google Maps. They should align.

Now Google Maps is WGS84, so it all depends on the datum and you cannot determine that from what you have.

How old are the co-ords. That might give you an idea as to when they were taken (assume they were not updated) and may give you a hint as to the datum.

The most likely meaning of those figures is that the first three are degrees, minutes and seconds north of the Equator (if they were negative, they would be in the Southern Hemisphere), and the second three are degrees, minutes and seconds east of the Greenwich Meridian (and since they are all negative, they are in the Western Hemisphere).

These are, as noted, lat/long coordinates, Degrees/Minutes/Seconds. It is absolutely impossible to tell by coordinates alone whether these are NAD27, NAD83, or WGS84 just from the coordinates themselves, and without prior knowledge, there isn’t a definite way to tell.

In my neck of the woods, the difference between NAD27 and NAD83/WGS84 usually corresponds to about 150 feet or so apart.

VoluntaryFireman, GIS Analyst/Surveyor

Are they supposed to point to anything obvious? Can you plot them on a few satellite/aerial maps of different projections and see which one fits best?