Recording decimal latitude and longitude

[Robert K. Colwell]

Charles Hussey writes:

>>I am involved in setting up databases for recording specimen data at the
>>>Natural History Museum, London. We have provided separate fields for
>>>recording degrees, minutes, seconds and direction for latitude and
>>>longitude, plus fields for decimal degrees for lat. & long. to cope with
>>>output from GPS devices. In due course, in order to facilitate GIS
>>analysis >of data, we expect to convert degrees/minutes/seconds to
>>decimal degrees and >fill the decimal fields. >> >>In designing the
>>system I had anticipated that decimal degrees would be >presented as a
>>positive or negative number with up to four decimal places. >One of our
>>researchers, who uses a GIS device has alerted me to the fact >that his
>>GPS device outputs things as degrees and decimal minutes (114 >24.567'E
>>for example); which would indicate the requirement for an integer >degree
>>field and a decimal minute field. >> >>Can anyone out there who has
>>experience of these things provide details how >many formats exist for
>>data from GIS/GPS systems and which are the most common.



GIS: The lingua franca of spatial referencing in the GIS world is decimal
degrees, with positive sign for N and E, negative for S and W. For plotting
purposes and for computational and storage efficieny in database systems,
this makes excellent sense--like the metric system instead of yards,
feet,and inches makes sense.

GPS: Low-end and older GPS devices do indeed read out in integer degrees
and decimal minutes, although fancier and more recent ones can switch
between readouts in a variety of coordinate systems, including decimal
degrees, DMS, and others.

ATLASES: Beware! Some gazetteers list latitude and longitude values in DMS,
but use a format that looks like integer degrees with decimal minutes, e.g.
24deg 48.39 actually means 24deg 48min 39sec in these gazetteers. The
easiest way to detect this abomination is to see if you can find any values
in the gazetteer to the right of the "decimal point" that exceed 59. If
not, those values are *seconds*, not the decimal part of true decimal
minutes.

DATABASES: One solution with databasing spatial reference data (e.g. for
specimen collection localities) is to store all coordinates internally as
decimal degrees, then let the *interface* handle the conversions. My
biodiversity database mgt application, Biota (Sinauer Press), uses this
approach. The input screen for locality data presents all three common
systems (DMS;integer degrees plus decimal minutes; and decimal degrees).
The user can enter lat and long in any of the three systems, and the
coordinates are simultaneously displayed in all three systems in the input
screen as they are entered. For record listing (output) screens, either the
internal decimal degree format (fast) or DMS (a little slower, since
computation is required) is displayed, depending on a user preference
setting. Labels print in DMS.

Just to be sure we all mean the same thing, here are the conversion formulas.

DMS to decimal degrees, and to integer degrees plus decimal minutes:


Decimal Degrees = Degrees + (Minutes/60) + (Seconds/3600).
Integer Degrees = Degrees
Decimal Minutes = Minutes + (Seconds/60)

Decimal degrees to integer degrees plus decimal minutes, and to DMS:


AbsDD = Abs (Decimal Degrees)
Degrees = Int (AbsDD)
Decimal Minutes = (AbsDD - Degrees) * 60
Minutes = Int (Decimal Minutes)
Decimal Seconds = (Decimal Minutes - Minutes ) * 60
Seconds = Round (Decimal Seconds)

Regards to all,

Rob


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Robert K. Colwell, Professor
Department of Ecology and Evolutionary Biology, U-42
University of Connecticut
Storrs, CT 06269-3042
Tel: 860-486-4395   Fax: 860-486-3790  Internet: <colwell at uconnvm.uconn.edu>
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