← Previous → Next Contents
About harmonic constants and sub station corrections
Change log

Appendix B — Application of offsets for Min Flood and Min Ebb events

B.1  Background
B.2  Secondary Station Adjustments Instructions (NOS)
B.3  Comparison of old and new results
B.4  Comparison with published tables

Background

XTide distinguishes the following common events for current stations:

Max FloodMaximum current in the flood (+) direction.
Max EbbMaximum current in the ebb (−) direction.
Slack, Flood BeginsZero current preceding flood.
Slack, Ebb BeginsZero current preceding ebb.

Additionally, it distinguishes two events that are not seen as frequently:

Min FloodMinimum current in the flood (+) direction between two Max Floods when the current never crosses zero.
Min EbbMinimum current in the ebb (−) direction between two Max Ebbs when the current never crosses zero.

In XTide 2.8, a change was made to the application of offsets for Min Flood and Min Ebb events.

EventXTide 2.7 time adjustXTide 2.7 current adjustXTide 2.8 time adjustXTide 2.8 current adjust
Min FloodSame as Max EbbSame as Max Ebb Same as Slack, Flood Begins; if null, use Max FloodSame as Max Flood
Min EbbSame as Max FloodSame as Max Flood Same as Slack, Ebb Begins; if null, use Max EbbSame as Max Ebb

This change was made based on a reading of the highlighted sections of the National Ocean Service web page quoted below, which was downloaded from http://co-ops.nos.noaa.gov/currents04/t2chelp.html on 2004-09-15.

The second highlighted passage states that no attempt is made to predict the speed of the minimum currents.  It would not make sense for XTide to leave these values unadjusted because a very small ratio could cause the supposed maxima to have lower amplitude than the minimum.  Applying the ratio used for the surrounding maxima will give reasonable looking results as long as the adjustment is only a ratio.  (Additive adjustments would not produce reasonable results.)


BEGIN NATIONAL OCEAN SERVICE TEXT (highlighting added)

Secondary Station Adjustments Instructions

The publication of full daily predictions is necessarily limited to a comparatively small number of stations. These stations are referred to as "reference stations". Tidal current predictions for more than 2500 other locations can be obtained by applying certain differences to the daily predictions for the reference stations.

These pages provide a listing of the more than 2500 "subordinate stations" for which such predictions can be made, the differences and ratios to be used, and a link to the appropriate reference station. The stations in the listing are arranged geographically to make it possible to find stations which are available for an area you are interested in.

Since all tidal current stations are located offshore, many of them are named for the channels, rivers, and inlets they are located in, or for cities, towns, or navigational points they are located near. Some personal knowledge of the area you are interested in may be necessary to determine which station(s) are most appropriate for your use.

Depths: Although current measurements may have been recorded at various depths in the past, the data listed here for most subordinate stations are mean values determined to have been representative of the current at each location. For that reason, no specific current meter depth for those stations are given. Beginning with the Boston Harbor tidal current survey in 1971, data for individual meter depths were published and subsequent new data may be presented in a similar manner.

Since most of the current data in these pages came from meters suspended from survey vessels or anchored buoys, the listed depths are those measured downward from the surface. Some later data have come from meters anchored at fixed depths from the bottom. These meter positions were defined as depth below chart datum. Such defined depths in these pages will be accompanied by the small letter "d".

Minimum Currents: The user may note that at many locations the current may not diminish to a true slack water or zero speed stage. For that reason, the phrases, "minimum before flood" and "minimum before ebb" are used in these pages rather than "slack water" although either or both minimums may actually reach a zero speed value at some locations.

Maximum Currents: Near the coast and in inland waters, the current increases from a minimum current (slack water) for a period of about 3 hours until the maximum speed or strength of the current is reached. The speed then decreases for another period of about 3 hours when minimum current is again reached and the current begins a similar cycle in the opposite direction. The current that flows towards the coast or up a stream is known as the flood current; the opposite flow is known as the ebb current. Speeds of the current at reference stations are listed as positive values for floods and negative values for ebbs. These pages list the average directions of the maximum floods and maximum ebb currents. The directions listed are given in degrees, true, reading from 000 at north to 359 and are the directions toward which the current flow.

Differences and Speed Ratios: These pages contain time differences by which the user can compile approximate times for the minimum and maximum current phases at the subordinate stations. Time differences for those phases should be applied to the corresponding phases at the reference station. It will be seen upon inspection that some subordinate stations exhibit either a double flood or a double ebb stage, or both. In those cases, a separate time difference is listed for each of the three flood (or ebb) phases and should be applied only to the maximum flood (or ebb) phase at the reference station. The results obtained by the application of time differences will be based upon the local time meridian. Differences of time meridians between a subordinate stations and its reference station have been accounted for.

The speed ratios are used to compile approximations of the daily current speeds at the subordinate stations and refer only to the maximum floods and ebbs. No attempt is made to predict the speed of the minimum currents. These ratios are multiplied to the corresponding maximum current phases at the reference station. As mentioned before, however, some stations may exhibit either a double flood or a double ebb, or both. As with time differences, separate ratios are listed for each of the three flood (or ebb) phases and should be applied only to the daily maximum flood (or ebb) speed at the reference station. It should be noted that although the speed of a given current phase at a subordinate station is obtained by reference to the corresponding phase at a reference station, the directions of the current at the two places may differ considerably. These pages list the average directions of the maximum current phases at the subordinate stations.

Example Tidal Current Calculations

For Cape May Channel, the time and speed adjustments listed in the tables are:

Minimum       Minimum         Speed
Before Flood  Before  Ebb     Ratio
Flood         Ebb           Flood Ebb
-1 14  -1 30  -1 11  -0 45   1.1  1.8

and the reference station is Delaware Bay Entrance. If the times and speeds listed in column 1 are the minimum and maximum tidal currents for a day at Delaware Bay Entrance, column 2 are the time corrections, and column 3 are the speed corrections; column 4 will be the predicted currents at Cape May Channel. These values are computed by adding or subtracting the times in column 1 to the adjustments in column 2; and by multiplying the speeds in column 1 by the ratios in column 3.

     (1)                 (2)        (3)         (4)
  Times     Speed                            Times     Speed
0114  0425   1.3    -1 14  -1 30   *1.1    0000  0255   1.4
0736  1055  -1.3    -1 11  -0 45   *1.8    0625  1010  -2.3
1351  1650   1.2    -1 14  -1 30   *1.1    1237  1520   1.3
1958  2316  -1.3    -1 11  -0 45   *1.8    1847  2231  -2.3

END NATIONAL OCEAN SERVICE TEXT

Comparison of old and new results

From http://co-ops.nos.noaa.gov/currents04/tab2pc2.html (2004-09-15):

                                                              Minimum       Minimum         Speed    Direction
                                                              Before Flood  Before  Ebb     Ratio    At Maximum
Station                                              Depth    Flood         Ebb           Flood Ebb  Flood Ebb  Reference Station
Admiralty Head, 0.5 mile west of                              -0 31  -0 03  +0 01  -0 07   1.3  1.2   145  025  Admiralty Inlet

Resulting XTide data set:

NameAdmiralty Head, 0.5 mile west of, Washington Current
ReferenceAdmiralty Inlet, Washington Current
Max time add-00:03
Max level addNULL
Max level mult1.300
Min time add-00:07
Min level addNULL
Min level mult1.200
Flood begins-00:31
Ebb begins+00:01

NOS predictions for 2004-09-08 and 2004-09-09 at reference station, from http://co-ops.nos.noaa.gov/currents04/ADMIRALI.shtml (2004-09-15):

    Slack    Maximum   Slack    Maximum   Slack    Maximum   Slack    Maximum   Slack    Maximum
    Water    Current   Water    Current   Water    Current   Water    Current   Water    Current
Day  Time  Time  Veloc  Time  Time  Veloc  Time  Time  Veloc  Time  Time  Veloc  Time  Time  Veloc
     h.m.  h.m.  knots  h.m.  h.m.  knots  h.m.  h.m.  knots  h.m.  h.m.  knots  h.m.  h.m.  knots

  8         402   -2.3   814  1122    1.5  1449  1801   -1.2        2257   -0.1
  9         508   -2.4   909  1219    1.7  1543  1856   -1.5

Corresponding XTide results:

Reference stationSub station (XTide 2.7) Sub station (XTide 2.8)
2004-09-08  4:02 AM PDT  -2.33 knots  Max Ebb
2004-09-08  8:13 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-08 11:22 AM PDT   1.51 knots  Max Flood
2004-09-08  2:48 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-08  6:01 PM PDT  -1.22 knots  Max Ebb
2004-09-08 10:57 PM PDT  -0.07 knots  Min Ebb
2004-09-09  5:08 AM PDT  -2.36 knots  Max Ebb
2004-09-09  9:08 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-09 12:19 PM PDT   1.71 knots  Max Flood
2004-09-09  3:42 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-09  6:56 PM PDT  -1.47 knots  Max Ebb
2004-09-09 11:22 PM PDT   0.00 knots  Slack, Flood Begins
2004-09-08  3:55 AM PDT  -2.80 knots  Max Ebb
2004-09-08  7:42 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-08 11:19 AM PDT   1.96 knots  Max Flood
2004-09-08  2:49 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-08  5:54 PM PDT  -1.47 knots  Max Ebb
2004-09-08 10:54 PM PDT  -0.09 knots  Min Ebb
2004-09-09  5:01 AM PDT  -2.84 knots  Max Ebb
2004-09-09  8:37 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-09 12:16 PM PDT   2.22 knots  Max Flood
2004-09-09  3:43 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-09  6:49 PM PDT  -1.77 knots  Max Ebb
2004-09-09 10:51 PM PDT   0.00 knots  Slack, Flood Begins
2004-09-08  3:55 AM PDT  -2.80 knots  Max Ebb
2004-09-08  7:42 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-08 11:19 AM PDT   1.96 knots  Max Flood
2004-09-08  2:49 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-08  5:54 PM PDT  -1.47 knots  Max Ebb
2004-09-08 10:58 PM PDT  -0.08 knots  Min Ebb
2004-09-09  5:01 AM PDT  -2.84 knots  Max Ebb
2004-09-09  8:37 AM PDT   0.00 knots  Slack, Flood Begins
2004-09-09 12:16 PM PDT   2.22 knots  Max Flood
2004-09-09  3:43 PM PDT  -0.00 knots  Slack, Ebb Begins
2004-09-09  6:49 PM PDT  -1.77 knots  Max Ebb
2004-09-09 10:51 PM PDT   0.00 knots  Slack, Flood Begins

Comparison with published tables

When the change in XTide's behavior was made in 2004, the NOS web site did not provide calculated predictions at the subordinate stations for comparison.  Upon reviewing the issue in 2007 (at which time those predictions were available), it was found that the published tables did neither of the behaviors that were implemented in XTide.

Time offsets applied to Min Ebb event
XTide 2.7XTide 2.8NOS 2007
FloodMinimum Before EbbEbb

Whereas the behavior of the published tables seemed to be in conflict with the Secondary Station Adjustments Instructions, it was resolved not to change the behavior of XTide at that time.

Reference station

NOS table copied from http://tidesandcurrents.noaa.gov/get_predc.shtml?year=2007&stn=6030+Admiralty+Inlet&fldavgd=179&ebbavgd=003&footnote= 2007-02-24

     Admiralty Inlet
Predicted Tidal CurrentMarch, 2007
Flood Direction, 179 True.Ebb (-)Direction, 003 True.
NOAA, National Ocean Service
 Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
DayTime
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
1200490347-0.9 0825-0.1 1458-2.419172229+1.5
1302070512-1.0 0948-0.2 1610-2.420192335+1.8

Comparable XTide output, using harmonics-rmk-20040615.tcd
tide -l"admiralty inlet" -b"2007-03-12 00:00" -e"2007-03-14 00:00" -mc -empSsMm -tf"%H%M" -fh

DaySlackFlood

Slack
Ebb
SlackFlood

Slack
Mon 1200480347 −0.93 kt
0824 −0.08 kt
1457 −2.41 kt
19162229 1.52 kt
Tue 1302060512 −1.04 kt
0948 −0.18 kt
1610 −2.45 kt
20182335 1.75 kt

Subordinate station

Name             Agate Pass, North End of, Washington Current
Reference        Admiralty Inlet, Washington Current
Min time add     -0:59
Min level add    NULL
Min level mult   0.700
Max time add     -1:00
Max level add    NULL
Max level mult   0.800
Flood begins     -1:28
Ebb begins       -0:18

NOS table copied from http://tidesandcurrents.noaa.gov/get_predc.shtml?year=2007&stn=6030+Admiralty+Inlet&secstn=Agate+Passage,+north+end&sbfh=%2D1&sbfm=28&fldh=%2D1&fldm=00&sbeh=%2D0&sbem=18&ebbh=%2D0&ebbm=59&fldr=0.8&ebbr=0.7&fldavgd=230&ebbavgd=032&footnote= 2007-02-24

     Agate Passage, north end
Predicted Tidal CurrentMarch, 2007
Flood Direction, 230 True.Ebb (-)Direction, 032 True.
NOAA, National Ocean Service
 Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
Slack
Water
Maximum
Current
DayTime
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
Time
h.m.
Time
h.m.
Veloc
knots
1200310248-0.6 0726-0.1 1359-1.717492129+1.2
1301490413-0.7 0849-0.1 1511-1.718512235+1.4

Comparable XTide output, using harmonics-rmk-20040615.tcd
tide -l"Agate Pass, North End of, Washington Current" -b"2007-03-12 00:00" -e"2007-03-14 00:00" -mc -empSsMm -tf"%H%M" -fh

DaySlackFlood

Slack
Ebb
SlackFlood

Slack
Mon 1200300248 −0.65 kt
0806 −0.06 kt
1358 −1.69 kt
17482129 1.21 kt
Tue 1301480413 −0.73 kt
0930 −0.13 kt
1511 −1.71 kt
18502235 1.40 kt


← Previous → Next Contents
About harmonic constants and sub station corrections
Change log