I have been given a spreadsheet containing Degrees Minutes Seconds coordinates in the format 'DD MM SSS' (including the spaces). I've used the DMS - DD formula ‘DD = (Seconds/3600) + (Minutes/60) + Degrees' to convert the DMS values to DD. I have also used the Convert Coordinate Notation tool in ArcMap. Both methods agree that my DMS of 53° 47’ 985” = 54.05694. This would seem to be because the seconds contain three digits when they should contain only two. Does anyone have any ideas as to how this could have arisen? I'm now thinking that what I have is in fact Degrees, Minutes, Decimal Seconds.

well more than that is wrong since there is no way of getting 54 degrees in any event.

consider 54 deg 54 min 54 seconds

54 deg 54 +54/60 min

54 deg 54.90 min

54 deg 54.9/60

notice it never gets to 55, no matter how hard you try.

quit trying to use those old formula since they lead to errors, just use sequential math and you will notice that you don't 'add' to the degree portion it 'becomes' the decimal portion a conceptual versus mathematical difference