Calculating current velocities from raw data

Hi,
I have some binary data files from a current meter attached to a survey ship and I need to calculate the current speed and direction. The data is recorded with 3 beams and 15 cells, and the coordinate system is XYZ. I have separate files of the ships position and heading. How do I calculate the current speed and direction from the data in the data field for velocity - I understand this is stored as Vel[#cells][#beams].

I need to know the way the XYZ data is sored, ie. is it
Vel[Cell-1][X]
Vel[Cell-1][Y]
Vel[Cell-1][Z]
etc

or

Vel[Cell-1][X]
Vel[Cell-2][X]
.
.
Vel[Cell-15][X]

Once I know which elements of the array structure hold the XYZ for each cell, then I need to know how to calculate the velocity from the XYZ values.

Oh, and how do I tell what depth the 15 cells are measuring at.

thanks
Sean.

Dear Sean

What kind of current profiler do you use?

- Atle Lohrmann

Thanks Attle,

All I know is that it is a Nortek ADP current profiler, it's fitted to a seismic survey ship. I have the binary data from it which gives the serial number as N4639 if that helps.

I am not on the ship, I have been passed the data and asked to merge it with navigation data to build an attribute database. As it happens, I used exactly the same current meter on a survey 18 months ago, but on that occsion I interfaced directly to the data-stream from the meter via RS232 and had it set to ASCII. On that occasion the data looked like this, I include it here in case it helps you to help me. thanks again, Sean.

  16       N4639       51830 2002  5 10 12 38 40 3 0 0 0 2
   20   100    50     5     9
 2555    11   -21  6000  2282     0     0     0     0     0 15495
0  30343633  70692133   -56  -203   232  1632   1   0  44  36  32   0  88
 1  -2434    545     94   2462    387    123    -97    311    155    205   -260    146 123 148  97
 2  -2232    658     83   2316    208    104     84    424    153     77   -432    136  85 114  77
 3  -2208    707     92   2312    153    111    106    473    163     75   -486    144  72  72  67
 4  -2223    700     77   2323    166     96     92    466    153     85   -474    134  65  58  57
 5  -2236    607     98   2301    256    121     81    372    170     61   -383    155  57  54  52
 6  -2191    640    111   2271    208    132    122    405    183     35   -429    166  52  51  46
 7  -2165    585     96   2227    250    118    144    350    169     -6   -386    153  48  46  41
 8  -2046    522    165   2093    262    187    252    286    239   -130   -369    223  46  43  38
 9  -2102    568    133   2162    241    155    201    333    200    -65   -392    184  46  40  36
10  -2005    486    147   2041    281    170    289    249    215   -178   -347    200  45  40  34
11  -1977    331    254   1958    410    282    314     89    311   -261   -212    301  45  38  33
12  -1629    618    193   1740     18    203    628    380    274   -445   -597    247  44  37  33
13  -1883    474    301   1923    241    321    400    231    365   -289   -378    348  44  37  33
14   -714   -144    169    609    389    188   1452   -388    194  -1494   -183    179  44  36  32
15   -730   -164    462    615    401    482   1438   -416    473  -1493   -163    459  44  36  32
16    548    273    511   -411   -477    489   2596     18    549  -2409   -997    495  44  36  32
17  -1186    247    684   1189    178    698   1031    -21    752   -971   -392    731  43  36  31
18   1173  -1264    122  -1557    736    144   3150  -1501     96  -3483    229     87  44  36  31
19    169     87    348   -126   -157    340   2251   -164    380  -2155   -693    340  45  36  32
20    370  -1663    502   -960   1386    555   2429  -1903    481  -2963    852    502  44  36  32

Hi

What you have here is what we call "Extended ASCII format" for vessel mounted current profilers.  This is a format that is generated by the SurveyP software as a special serial output option.

The format is documented in the VM-NDP manual and in the Survey online help.  Below you will find the key specifications in case you do not have the manual or the software:

Extended ASCII format

Header lines 1-3 are as for the standard ASCII output.
Header line 4:
Field 1: GPS coordinate system (0=geog; 1=map),
Field 2: GPS latitude (in 0.0000001 deg) or northing (in 0.01 m)
Field 3: GPS longitude (in 0.0000001 deg) or easting (in 0.01 m)
Field 4: vessel north speed (in 0.01 m/s)
Field 5: vessel east speed (in 0.01 m/s)
Field 6: vessel water speed (in 0.01 m/s)
Field 7: vessel water speed direction (in 0.1 deg)
Field 8: water speed coordinate system (1=XYZ, 2=ENU)
Field 9: correction mode (0=DGPS, 1=bottomtrack)
Fields 10-14: system variables

Column 1: depth cell number
Column 2: raw velocity data (Vx) in mm/s
Column 3: raw velocity data (Vy) in mm/s
Column 4: raw velocity data (Vz) in mm/s
Column 5: raw velocity data (Veast) in mm/s
Column 6: raw velocity data (Vnorth) in mm/s
Column 7: raw velocity data (Vup) in mm/s
Column 8: corrected velocity data (Vx) in mm/s
Column 9: corrected velocity data (Vy) in mm/s
Column 10: corrected velocity data (Vz) in mm/s
Column 11: corrected velocity data (Veast) in mm/s
Column 12: corrected velocity data (Vnorth) in mm/s
Column 13: corrected velocity data (Vup) in mm/s
Column 14: amplitude (signal strength) data (beam 1) in counts (1 count = 0.43 dB)
Column 15: amplitude (signal strength) data (beam 2) in counts (1 count = 0.43 dB)
Column 16: amplitude (signal strength) data (beam 3) in counts (1 count = 0.43 dB)  

Best regards,

Atle Lohrmann

thanks Atle,

sorry, I didn't make myself clear. The I included the ASCII format above to help you identify the type of meter. The data I am parsing now is the binary version from the same meter. I have written a parser which strips the data out, but I need to know how to calculate the actual current data from the XYZ velocity data. So if I have Vx, Vy, and Vz for a cell is it simply a case of summing the vectors?  My real interest is in currents in X/Y plane, and in different currents at different depths.

Also, my data has 15 cells, which I take it is 15 different depths. How do I determine the depth of each cell?

thanks again,
Sean.

Attached is the NDP binary format, for the Nortek NDP (*.adp) file. This is further documented in the Help files and Manual under 'Data Formats'.
The format includes the Vx,Vy and Vz which is the sum of the vessel and water current velocities, to get the water current  the x, y and z velocities of the vessel have to be subtracted. You then have the water current in X,Y and Z which can be simply transformed in to a resultant current speed and direction using pythagoras and simple trigonometry.

Quote

Also, my data has 15 cells, which I take it is 15 different depths. How do I determine the depth of each cell?

The format description explains that after the initial header information, all the cells are listed after each other,
typedef struct {
  int            Vel[MAX_CELLS][MAX_BEAMS];      // in 1 mm/s
  unsigned char  VelStd[MAX_CELLS][MAX_BEAMS];   // in 1 mm/s
  unsigned char  Amp[MAX_CELLS][MAX_BEAMS];
} ProfileType;
The first cells starts at a depth corresponding to 1 cell size + blanking + your mounting depth. The second cell is this + 1 cell, etc.

thanks Paul,

that was the bit I was missing, subtracting the vessel velocities. No wonder the answers I was getting weren't makng sense.

cheers
Sean.