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» Online Classroom   »   » Public Discussion of Cel Nav   » Starpath Style Sheet

   
Author Topic: Starpath Style Sheet
David Burch


 - posted May 24, 2003 11:18 PM      Profile for David Burch           Edit/Delete Post 
Starpath Style Sheet and Conventions

These are a few conventions and styles we try to adhere to in our publications, but have noticed that we are not completely consistent. So we put these up front as much as a reminder to ourselves. Some are obviously just a matter of style, some have more practical significance. There is also at the end some explanation of how we use a few specific terms.

Abbreviations and labels use no periods
minutes = min or m
hours = hr or h
seconds = sec or s

knots = kts ie 4.5 kts 0.5 kts
nautical miles = nmi 0.5 nmi 346 nmi

Numbers format
for values less than one, always include leading zero. ie 0.5' not .5', etc

Special characters
to type a degree sign(°), turn on Num Lock, then hold down Alt key and type 0176.
to type a ± sign, turn on Num Lock, then hold down Alt key and type 0177.

Lat and Lon
47° 05.6' W, 122° 33.5' E
7° 15.6' W, 22° 03.5' E
space between degrees and minutes, leading 0 on minutes but not degrees, label N, S goes after one space.

Bearings, azimuth, azimuth angle, course headings, etc
345° T, 047° M, 325° or 325.5°
leading 0 for under 100, space between number and label

Sextant readings
34° 32.5' or 5° 04.2'
leading 0 on minutes, but not degrees

Times
04h 03m 32s WT
13h 12m 54s UT, etc
leading 0 all values, space between h m s, no space between number hms label, but use a space before time system label
or
12:04:05 WT etc. all times always have a label, GMT, Z, or UT for GMT, or WT, or PDT, etc

for DR positions
0404 WT or 1614 WT etc, no seconds given. For a given log book or specific problem, might omit label if unambiguous.

Zone descriptions, etc from this definition
GMT = WT + ZD or WT = GMT - ZD,
hence ZD = +5 omit hr label, no leading zeros
East longitude time zones have negative ZDs, West longitudes positive. Lon 120° W or PST has ZD = +8,

Equations
ZD = +4, Hs = 32° 33.1'
always a space either side of = sign

Speeds
S = 7.5
give to nearest tenth

Courses
C = 320 M or 029.4 T
give to nearest degree, unless in special circumstances such as precise solution to a problem

SOG = speed over ground, an instantaneous value, can be read from GPS
SMG = speed made good, the distance between any two points on a track divided by the time difference between the two points. Cannot be read from GPS, but can be computed from a logbook. This can be a past computation, or a predicted one. It is independent of the past or predicted route between the two points.

COG = course over ground, an instantaneous value, can be read from GPS
CMG = course made good, the direction between any two points from your past track. Cannot be read from GPS as output, but can be computed from a logbook, or extracted from e-chart plot.

SOA = speed of advance, an instantaneous value, can be read from GPS. It is your present SOG projected in the direction of the next waypoint. It is simply how fast you are getting to where you want to go. Most GPS units use this concept to compute the ETA. We must be careful with this term. Some GPS units call this VMG, which has some legitimacy, although still unfortunate since the term is used now and has been used by sailors for many years to refer to the wind direction, not a navigation direction. Also note, Bowditch has a different definition of this term related to planning a ship route for arrival at a particular time—they use SOA to mean a predicted SMG you need make along a specific track to make a specific arrival time. In small craft applications, we might say “If we follow this route we have to make good a speed of 7.5 kts to get there by sunset.” …and let it go at that, rather than use a special term for this speed which conflicts with modern electronic outputs. In the end, however, we are at the mercy of the IMO and NMEA to set the standards on the terms we must use. I don’t think this one is sorted out yet.

VMG = knotmeter speed projected in the direction of the true wind. This is a sailing vessel performance value, not really related to navigation. Again, for now this is what we prefer and recommend, but we must wait on the IMO and NMEA, among others to tell us what to call these things. Right at the moment, the actual electronics manufacturers have more freedom in this arena than is best for the overall progress of the terminology. They must use the right conventions in the electronic outputs (NMEA) sent to other devices, but what they call the quantities on the dials, screen displays, and manuals is still up to them. With many vying for the winning name… ie one GPS company calls SOG the “ground speed” instead of speed over ground. Some call it track speed, some call it just speed (worst of all), etc.

From: Starpath, Seattle, WA


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