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Converting unusual lat/long coordinates to 'standard'

I realise there are more important happenings in the world at the moment, but while house-bound, and finding diversions, I’m fine-tuning some Kenyan co-ordinates, just issued by the KCAA for VFR routes in the Nairobi CZ, and plotting on Sky Demon.

One position (Nairobi race-course north-west of Wilson airfield) has been published as:

036°44’43.941"E 01°18’38.876"S

Am I correct in thinking that 0.001" is around 3cms latitude?

Swanborough Farm (UK), Shoreham EGKA, Soysambu (Kenya), Kenya

I’d say half a cm? The earth’s circumference is about 3600 nmi, making one degree of latitude (or one degree of longitude at the equator) about 10 nmi, a minute 1/6 nmi = 300 m, a second about 5 m and a thousandth of a second 5 mm.

Last Edited by lionel at 25 Mar 14:22
ELLX

It is a small world, but not quite that small…

Biggin Hill

In the same vein, this wonderful booklet includes the following NOTAM:

FMSD MADAGASCAR A1154/17 – ‘COLLINE’ PSN 250029S 0465754E OBST ALT: – READ ‘1732FT’ INSTEAD OF ‘1733FT’.

Last Edited by Airborne_Again at 25 Mar 14:40
ESKC (Uppsala/Sundbro), Sweden

Yes.

The linear element of meridian arc length is ds = R(φ) dφ. R is the radius of curvature of the meridional ellipse, dependent on the semi-major axis length and first eccentricity of the ellipsoid, and φ is latitude. Assuming a constant radius a yields s = a Δφ. Using a figure of 6 378 137.0 m for the radius (suitable near the equator) gives about 3.1 cm per thousandth second of arc. For R(φ) see eqn 154 in Thomas, P. D. (1952). Conformal Projections in Geodesy and Cartography. Special Publication No. 251. Coast and Geodetic Survey, US Department of Commerce (pdf link).

London, United Kingdom

Cobalt wrote:

It is a small world, but not quite that small…

Argh, you are right, I got the wrong numbers. The cirumference is about 40000 km = about 21600 nmi = 360*60 nmi, making one minute of longitude about 1 nmi, a second about 31 m and a thousandth of a second 31 mm. Sounds more in line with the PPL courses :)

ELLX

There are so many formats for latitude and longitude, there is no real standard. If there is a standard format, who follows it? Latitude and Longitude is basically measured in degrees or degrees and minutes, or degrees and minutes and seconds and combinations of these with decimal values. The hemispheric value can be expressed as N/S or e/W in front of or after the number or by a sign. So if you are not adept at understanding the different possibilities of representing exactly the same thing, it is easy to fool your self. So my house is at

35 09.17N 080 47.97W or N35 09.17 W80 47.97 in DD MM.mm format where DD is degrees, MM is minutes and mm is hundredths of a minute. One can add more precision by adding more decimals of minutes. Most defaults for GPS use some variant of this format as the default format.

Those that use numbers in calculations will use DD.ddddd where DD is degrees and ddddd is decimal degrees. My address is:
35.15301 -80.79929 where the sign is – for W and S.

In ForeFlight, it displays Latitude and Longitude in three basic formats where DD is degrees, dd is decimal degrees, MM is minutes, mm is decimal minutes, SS is seconds and H is hemispheric character:
Format 1 DD.dd H (35.15N 80.80W)
Format 2 DD MM.mm H (
Format 3 DD MM SS H (35 09 11N 80 47 57W)

Of these formats, the precision of the latitude is to the nearest 3640 feet for format 1, format 2 has the greatest latitude precision of 60 feet. and Format 3 is to the nearest 100 feet. I use Format 2 because it has the best precision that ForeFlight will display and is easy to match up with the default position on my GTN750 which uses the same format, but precedes the number with the H value. My GPSMAP uses a similar format but adds a third decimal digit value (H DD MM.mmm).

So when figuring out where a particular system is pointing to, determine where the decimal is. If it is after degrees DD.dd the dd is decimal degrees) if it is after the minutes, then the format is a variation of DD MM.mm. If it is after seconds, then the format DD MM SS.ss is decimal seconds. Then figure out how the hemispheric value is determined, by N/S/E/W in front or after the value or is a sign used for this purpose.

Then decide which format you want to use and convert the decimal or whole values in the source data as appropriate. There are 60 seconds in 1 minute and there are 60 minutes in one degree. If there is a sign used, the convention is S is minus and all W values are minus.

Other than that everything is standard unless you use a different system all together such as MGRS. :)

IOW the standard as a practical matter does not exist and you are likely to need to convert and know how to multiply and divide by 60.

KUZA, United States

Has anyone written a converter which extracts the right values, by a process of elimination?

I am surprised google have not done that, for e.g. google maps.

Administrator
Shoreham EGKA, United Kingdom

ForeFlight accepts latitude longitude in a large variety of formats and converts it to a decimal format internally. It only supports display in three formats.

KUZA, United States

Peter wrote:

I am surprised google have not done that, for e.g. google maps.

They are apperently accepting different formats:

https://support.google.com/maps/forum/AAAAQuUrST8BMRtlZaGEW8

Open Google Maps.
In the search box at the top, type your coordinates. Here are examples of formats that work:
Degrees, minutes, and seconds (DMS): 41°24’12.2"N 2°10’26.5"E
Degrees and decimal minutes (DMM): 41 24.2028, 2 10.4418
Decimal degrees (DD): 41.40338, 2.17403
You’ll see a pin show up at your coordinates.
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