/usr/share/doc/gmt/html/man/mgd77sniffer.html

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<title>MGD77SNIFFER</title>

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<h1 align="center">MGD77SNIFFER</h1>

<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#REQUIREMENTS">REQUIREMENTS</a><br>
<a href="#OPTIONS">OPTIONS</a><br>
<a href="#MGD77 FIELD INFO">MGD77 FIELD INFO</a><br>
<a href="#GRID FILE INFO">GRID FILE INFO</a><br>
<a href="#E77 ERROR FORMAT">E77 ERROR FORMAT</a><br>
<a href="#EXAMPLES">EXAMPLES</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<a href="#REFERENCES">REFERENCES</a><br>

<hr>


<h2>NAME
<a name="NAME"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em">mgd77sniffer
&minus; Scan for errors in MGD77 data</p>

<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>



<p style="margin-left:11%; margin-top: 1em"><b>mgd77sniffer</b>
<i>NGDC-ids</i> [
<b>&minus;A</b><i>fieldabbrev</i>,<i>scale</i>,<i>offset</i>
] [ <b>&minus;C</b><i>maxspd</i> ] [
<b>&minus;Dd</b>|<b>e</b>|<b>E</b>|<b>f</b>|<b>l</b>|<b>m</b>|<b>s</b>|<b>v</b>[<i>r</i>]
] [
<b>&minus;g</b><i>fieldabbrev</i>,<i>imggrid</i>,<i>scale</i>,<i>mode</i>
] [ <b>&minus;G</b><i>fieldabbrev</i>,<i>grid</i> ] [
<b>&minus;H</b>[<b>i</b>][<i>nrec</i>] ] [
<b>&minus;I</b><i>fieldabbrev</i>,<i>rec1</i>,<i>recN</i> ]
[ <b>&minus;K</b> ] [ <b>&minus;L</b><i>custom limits
file</i> ] [ <b>&minus;N</b> ] [
<b>&minus;Q</b>[<b>b</b>|<b>c</b>|<b>l</b>|<b>n</b>][[<b>/</b>]<i>threshold</i>]
] [
<b>&minus;R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
] [ <b>&minus;Sd</b>|<b>s</b>|<b>t</b> ] [
<b>&minus;T</b><i>gap</i> ] [ <b>&minus;V</b> ] [
<b>&minus;Wc</b>|<b>g</b>|<b>o</b>|<b>s</b>|<b>t</b>|<b>v</b>|<b>x</b>
] [
<b>&minus;bo</b>[<b>s</b>|<b>S</b>|<b>d</b>|<b>D</b>[<i>ncol</i>]|<b>c</b>[<i>var1</i><b>/</b><i>...</i>]]
]</p>

<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>



<p style="margin-left:11%; margin-top: 1em"><b>mgd77sniffer</b>
scans old (pre-Y2K) and new format ASCII MGD77 files for
errors using point-by-point sanity checking, along-track
detection of excessive slopes, and optional comparison of
cruise data with global gravity and predicted bathymetry
grids. Detected data problems are output by default as
verbose descriptions of each detected error, often resulting
in multiple messages per scanned record. Data problems are
optionally output (<b>&minus;De</b> option) using a
computer-parseable format (see E77 ERROR FORMAT description
below). Default error thresholds are derived from histograms
of all MGD77 geophysical data collected between 1952 and
January, 2006. Thresholds are adjustable with the
<b>&minus;L</b> option. <i><br>
NGDC-ids</i></p>

<p style="margin-left:22%;">Can be one or more of five
kinds of specifiers: <br>
1) 8-character NGDC IDs, e.g., 01010083, JA010010etc., etc.
<br>
2) 2-character &lt;agency&gt; codes which will return all
cruises from each agency. <br>
3) 4-character &lt;agency&gt;&lt;vessel&gt; codes, which
will return all cruises from those vessels. <br>
4) =&lt;list&gt;, where &lt;list&gt; is a table with NGDC
IDs, one per line. <br>
5) If nothing is specified we return all cruises in the data
base. <br>
(See mgd77info <b>&minus;L</b> for agency and vessel codes).
The &quot;.mgd77&quot; or &quot;.nc&quot; extensions will
automatically be appended, if needed (use <b>&minus;I</b> to
ignore certain file types). Cruise files will be looked for
first in the current directory and second in all directories
listed in <b>$MGD77_HOME</b>/mgd77_paths.txt [If
<b>$MGD77_HOME</b> is not set it will default to
<b>$GMT_SHAREDIR</b>/mgd77].</p>

<h2>REQUIREMENTS
<a name="REQUIREMENTS"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em">The
mgd77sniffer links with Generic Mapping Tools 4.0 or later
along with the supplemental GMT packages x2sys and mgd77.
See http://gmt.soest.hawaii.edu for GMT details. Grids for
comparison with cruise data may be downloaded via the
web.</p>

<h2>OPTIONS
<a name="OPTIONS"></a>
</h2>


<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;A</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">Apply scale factor and DC
adjustment to specified data field. Allows adjustment of
cruise data prior to along-track analysis. CAUTION: data
must be thoroughly examined before applying these global
data adjustments. May not be used for multiple cruises.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;C</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Set maximum ship speed in m/s, or knots with
<b>&minus;N</b> option. Ship speeds exceeding 10 m/s (~20
knots) are flagged as excessive by default.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;D</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Suppress default warning output and only dump cruise
data row-by-row such as values, gradients, grid-cruise
differences, E77 error summaries for each record, re-created
MGD77 records or sniffer limits. Append r to include all
records (default omits records where navigation errors were
detected).</p> </td></tr>
</table>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Dd</b>
output differences between cruise and grid data. Requires
<b>&minus;G</b> option. Output columns include:</p>

<p style="margin-left:22%; margin-top: 1em"><i>lat lon dist
cruiseZ gridZ diff [cruiseZ2 gridZ2 diff2 ...]</i></p>

<p style="margin-left:22%; margin-top: 1em">Note: grid
values are subtracted from cruise data so a positive
difference implies cruise &gt; grid. For multiple grid
comparison, <i>cruiseZ gridZ diff</i> are repeated for each
grid comparison in command line order.</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;De</b>
output E77 error classification format. Error output is
divided into (1) a header containing information globally
applicable to the cruise and (2) individual error records
summarizing all errors encountered in each cruise record.
mgd77sniffer writes E77 directly to &lt;ngdc_id.e77&gt; file
handle. See <b>E77 ERROR FORMAT</b> below for additional
details.</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;DE</b>
Same as <b>&minus;De</b> but no regression tests will be
carried out.</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Df</b>
output delta Z (change in geophysical field) column and
delta S (change in distance) for each geophysical field.
Distance between observations often differ for different
fields depending on instrument sampling rate, so ds is
included for each geophysical observation. Output columns
include:</p>

<p style="margin-left:22%; margin-top: 1em"><i>d[twt] ds
d[depth] ds d[mtf1] ds d[mtf2] ds d[mag] ds d[diur] ds
d[msd] ds d[gobs] ds d[eot] ds d[faa] ds</i></p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Dl</b>
display mgd77sniffer limits. Customize this output to create
a custom limits file for the <b>&minus;L</b> option. No
additional arguments are required. Output columns
include:</p>

<p style="margin-left:22%; margin-top: 1em"><i>fieldabbrev
min max maxSlope maxArea</i></p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Dm</b>
output MGD77 format records in Y2K-compliant MGD77
format</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Dn</b>
output distance to coast for each record. Requires
<b>&minus;Gnav</b> or <b>&minus;gnav</b> option. Output
columns include:</p>

<p style="margin-left:22%; margin-top: 1em"><i>lat lon dist
distToCoast</i></p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Ds</b>
output calculated gradients for speed and geophysical
fields. Gradients correspond to the gradient type selected
in the <b>&minus;S</b> option (spatial derivatives by
default). Output columns include:</p>

<p style="margin-left:22%; margin-top: 1em"><i>speed d[twt]
d[depth] d[mtf1] d[mtf2] d[mag] d[diur] d[msd] d[gobs]
d[eot] d[faa]</i></p>

<p style="margin-left:22%; margin-top: 1em">See <b>MGD77
FIELD INFO</b> below for field and abbreviations
descriptions.</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Dv</b>
display values for the twelve position and geophysical
fields for each MGD77 data record (in this order):</p>

<p style="margin-left:22%; margin-top: 1em"><i>lat lon twt
depth mtf1 mtf2 mag diur msens gobs eot faa</i></p>

<p style="margin-left:22%; margin-top: 1em">See below for
<b>MGD77 FIELD INFO</b>.</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;g</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">Compare cruise data to the
specified grid in Sandwell/Smith Mercator format. Requires a
valid MGD77 field abbreviation (see <b>MGD77 FIELD INFO</b>
below) followed by a comma, the path (if not in current
directory) and grid filename, a scale to multiply the data
(1 or 0.1), and mode which stand for the following: (0) Img
files with no constraint code, returns data at all points,
(1) Img file with constraints coded, return data at all
points, (2) Img file with constraints coded, return data
only at constrained points and NaN elsewhere, and (3) Img
file with constraints coded, return 1 at constraints and 0
elsewhere.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;G</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Compare cruise data to the specified grid. Requires a
valid MGD77 field abbreviation (see <b>MGD77 FIELD INFO</b>
below) followed by a comma, then the path (if not in current
directory) and grid filename. Multiple grid comparison is
supported by using separate <b>&minus;g</b> or
<b>&minus;G</b> calls for each grid. See <b>GRID FILE
INFO</b> below.</p></td></tr>
</table>

<p style="margin-left:22%; margin-top: 1em">Grid comparison
activates several additional error checks. (1) Re-weighted
Least Squares Regression of ship versus grid data determines
slope and DC shift, which when differing from expected 1 and
0, respectively, may indicate incorrectly scaled ship data,
including incorrect units or instrument drift as well as
erroneous gravity tie-in. (2) Accumulated ship grid offsets
are computed along-track and excessive offsets are flagged
according to <i>maxArea</i> threshold (use <b>&minus;L</b>
option to adjust <i>maxArea</i>). Warning: predicted
bathymetry grids are constrained by cruise data so grids and
cruise data are not always independent. Comparison of cruise
bathymetry with predicted bathymetry grids also activates a
&quot;navigation crossing over land&quot; check.</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;H</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">(with <b>&minus;G</b>|<b>g</b>
only) disable (or force) decimation during RLS analysis of
ship and gridded data. By default mgd77sniffer analyses both
the full and decimated data sets then reports RLS statistics
for the higher correlation regression.</p></td></tr>
</table>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Hb</b>
analyze both (default), report better of two.</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Hd</b>
to disable data decimation (equivalent to -H with no
argument).</p>


<p style="margin-left:22%; margin-top: 1em"><b>&minus;Hf</b>
to force data decimation.</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;I</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">Append a field abbreviation and
the first and last record in a range of records that should
be flagged as bad (and set to NaN prior to the analysis).
Repeat as many times as needed. May not be used for multiple
cruises.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;K</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Reverse navigation quality flags (good to bad and vice
versa). May be necessary when a majority of navigation fixes
are erroneously flagged bad, which can happen when a
cruise&rsquo;s first navigation fix is extremely erroneous.
Caution! This will affect sniffer output and should only be
attempted after careful manual navigation review.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;L</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Override mgd77sniffer default error detection limits.
Supply path and filename to the custom limits file. Rows not
beginning with a valid MGD77 field abbreviation are ignored.
Field abbreviations are listed below in exact form under
MGD77 FIELD INFO. Multiple field limits may be modified
using one default file, one field per line. Field min, max,
max slope and max area may be changed for each field. Max
slope pertains to the gradient type selected using the
<b>&minus;S</b> option. Max area is used by the
<b>&minus;G</b> option as the threshold for flagging
excessive offsets from the specified grid. Dump defaults
<b>&minus;Dl</b> to view syntax or to quickly create an
editable custom limits file.</p></td></tr>
</table>

<p style="margin-left:22%; margin-top: 1em">Example custom
default file contents (see below for units): <br>
# abbrev min max maxSlope maxArea <br>
twt 0 15 1 0 <br>
depth 0 11000 500 5000 <br>
mag -800 800 - - <br>
faa -300 300 100 2500</p>

<p style="margin-left:22%; margin-top: 1em">Use a dash
&rsquo;-&rsquo; to retain a default limit. Hint: to test
your custom limits, try: mgd77sniffer <b>&minus;Dl
&minus;L</b>&lt;yourlimitsfile&gt;</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;N</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">Use nautical units.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;P</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Flag regression statistics that are outside the
specified confidence level. (i.e., -P5 flags coefficients m,
b, rms, and r that fall outside 95%.)</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;Q</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Quick mode, use bilinear rather than bicubic
interpolation [Default]. Alternatively, select the
interpolation mode by adding <b>b</b> for B-spline
smoothing, <b>c</b> for bicubic interpolation, <b>l</b> for
bilinear interpolation or <b>n</b> for nearest-neighbor
value. Optionally, append <i>threshold</i> in the range
[0,1]. This parameter controls how close to nodes with NaN
values the interpolation will go. E.g., a <i>threshold</i>
of 0.5 will interpolate about half way from a non-NaN to a
NaN node, whereas 0.1 will go about 90% of the way, etc.
[Default is 1, which means none of the (4 or 16) nearby
nodes may be NaN]. <b>&minus;Q0</b> will just return the
value of the nearest node instead of interpolating. This is
the same as using <b>&minus;Qn</b>.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;R</b></p></td>
<td width="8%"></td>
<td width="78%">


<p><i>west, east, south,</i> and <i>north</i> specify the
Region of interest, and you may specify them in decimal
degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append
<b>r</b> if lower left and upper right map coordinates are
given instead of w/e/s/n. The two shorthands
<b>&minus;Rg</b> and <b>&minus;Rd</b> stand for global
domain (0/360 and -180/+180 in longitude respectively, with
-90/+90 in latitude). Alternatively, specify the name of an
existing grid file and the <b>&minus;R</b> settings (and
grid spacing, if applicable) are copied from the grid.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;S</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Specify gradient type for along-track excessive slope
checking.</p> </td></tr>
</table>

<p style="margin-left:22%;"><b>&minus;Sd</b> Calculate
change in z values along track (dz). Output is given in
geophysical units, e.g., mGal. <b><br>
&minus;Ss</b> Calculate spatial gradients (dz/ds). Output is
given in geophysical units per km along the survey track,
e.g., mGal/km. <b><br>
&minus;St</b> Calculate time gradients (dz/dt) [default].
Output is given in geophysical units per second along the
survey track, e.g., mGal/sec.</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p style="margin-top: 1em"><b>&minus;T</b></p></td>
<td width="8%"></td>
<td width="78%">


<p style="margin-top: 1em">Adjusts mgd77sniffer gap
handling. By default, data gaps greater than 5 km are
skipped. Set to zero to de-activate gap skipping.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;W</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Print out only certain warning types for verbose error
messages. Comma delimit any combination of
<b>c|g|o|s|t|v|x</b>: where (<b>c</b>) type code warnings,
(<b>g</b>)radient out of range, (<b>o</b>)ffsets from grid
(requires <b>&minus;G</b>|<b>g</b>), (<b>s</b>)peed out of
range, (<b>t</b>)ime warnings, (<b>v</b>)alue out of range,
(<b>x</b>) warning summaries. By default ALL warning
messages are printed.Not compatible with any <b>&minus;D</b>
options.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;V</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>Selects verbose mode, which will send progress reports
to stderr [Default runs &quot;silently&quot;].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">


<p><b>&minus;b</b></p></td>
<td width="8%"></td>
<td width="78%">


<p>output binary data for <b>&minus;D</b> d|f|s|v option.
Append <b>s</b> for single and <b>d</b> for double precision
[Default is double].</p></td></tr>
</table>

<h2>MGD77 FIELD INFO
<a name="MGD77 FIELD INFO"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em"><i>Field
Abbreviation Units</i> <br>
Two-way Travel Time twt sec <br>
Corrected Depth depth m <br>
Mag Total Field1 mtf1 nT <br>
Mag Total Field2 mtf2 nT <br>
Residual Magnetic mag nT <br>
Diurnal Correction diur nT <br>
Mag Sensor Depth/Alt msens m <br>
Observed Gravity gobs mGal <br>
Eotvos Correction eot mGal <br>
Free Air Anomaly faa mGal</p>

<h2>GRID FILE INFO
<a name="GRID FILE INFO"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em">For
<b>&minus;g</b> the grids must be in the format used by
Sandwell &amp; Smith, which is a spherical Mercator 2-byte
grid with no header. For <b>&minus;G</b> the grid files can
be of any grid type supported by GMT and therefore must
contain a GMT header. A correctly formatted *.i2 grid file
can be generated using grdraster as shown below.</p>

<p style="margin-left:11%; margin-top: 1em">gmtset
GRIDFILE_SHORTHAND TRUE</p>

<p style="margin-left:11%; margin-top: 1em">Create/edit
.gmt_io file to include the following rows: <br>
# GMT I/O shorthand file <br>
# suffix format_id scale offset NaN <br>
grd 0 - - - <br>
i2 2 - - 32767</p>

<p style="margin-left:11%; margin-top: 1em">grdraster 1
-R0/359:55/-90/90 -Getopo5_hdr.i2</p>

<p style="margin-left:11%; margin-top: 1em">The new grid,
etopo5_hdr.i2 in this example, contains a GMT header and can
be used in the <b>&minus;G</b> option to compare cruise
depth with grid values.</p>

<h2>E77 ERROR FORMAT
<a name="E77 ERROR FORMAT"></a>
</h2>


<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="9%">


<p style="margin-top: 1em"><b>Header</b></p></td>
<td width="2%"></td>
<td width="78%">


<p style="margin-top: 1em">Information pertaining to an
entire cruise, such as NGDC and survey institution
identification codes, cruise examination time, two-way
travel time corrector information, data precision warnings,
as well as systematic scales, DC shifts and correlation
coefficients from global grid comparisons are reported as
E77 header information.</p></td></tr>
</table>

<p style="margin-left:22%; margin-top: 1em"><b>Sample</b>
<br>
# Cruise 08010039 ID 74010908 MGD77 FILE VERSION: 19801230
N_RECS: 3066 <br>
# Examined: Wed Oct 3 16:30:13 2007 by mtchandl <br>
# Arguments: -De -Gdepth,/data/GRIDS/etopo5_hdr.i2 <br>
N Errata table verification status <br>
# mgd77manage applies corrections if the errata table is
verified (toggle &rsquo;N&rsquo; above to &rsquo;Y&rsquo;
after review) <br>
# For instructions on E77 format and usage, see
http://gmt.soest.hawaii.edu/mgd77/errata.php <br>
# Verified by: <br>
# Comments: <br>
# Errata: Header <br>
Y-E-08010039-H13-02: Invalid Magnetics Sampling Rate: (99) [
] <br>
Y-W-08010039-H13-10: Survey year (1975) outside magnetic
reference field IGRF 1965 time range (1965-1970) <br>
Y-I-08010039-depth-00: RLS m: 1.00053 b: 0 rms: 127.851 r:
0.973422 significant: 1 decimation: 0 <br>
Y-W-08010039-twt-09: More recent bathymetry correction table
available <br>
Y-W-08010039-mtf1-10: Integer precision <br>
Y-W-08010039-mag-10: Integer precision</p>

<p style="margin-left:11%;"><b>Error Record</b></p>

<p style="margin-left:22%;">Individual error records have
strict format. Included is a time or distance column
followed by record number, a formatted error code string,
and finally a verbose description of errors detected in the
record. Three error classes are encoded into the error code
string with different alphabetic characters representing
unique error types. See below for error code format
description.</p>

<p style="margin-left:22%; margin-top: 1em"><b>Format</b>
<br>
&lt;time/distance&gt; &lt;record number&gt; &lt;error code
string&gt; &lt;description&gt;</p>

<p style="margin-left:22%; margin-top: 1em"><b>Sample</b>
<br>
# Errata: Data <br>
Y 08010039 1975-05-10T22:16:05.88 74 C-0-0 NAV: excessive
speed</p>

<p style="margin-left:11%;"><b>Error Code
Description</b></p>

<p style="margin-left:22%;">Each of the three error classes
is separated by a dash &rsquo;-&rsquo; and described by a
combination of alphabetic characters or 0 signifying no
detected problems.</p>

<p style="margin-left:22%; margin-top: 1em">Error classes:
NAV-VAL-GRAD</p>

<p style="margin-left:22%; margin-top: 1em"><b>Error Class
Descriptions</b> <br>
NAV (navigation): <br>
0 - fine <br>
A - time out of range <br>
B - time decreasing <br>
C - excessive speed <br>
D - above sea level <br>
E - lat undefined <br>
F - lon undefined</p>

<p style="margin-left:22%; margin-top: 1em">VAL (value):
<br>
0 - fine <br>
K - twt invalid <br>
L - depth invalid <br>
O - mtf1 invalid <br>
etc.</p>

<p style="margin-left:22%; margin-top: 1em">GRAD
(gradient): <br>
0 - fine <br>
K - d[twt] excessive <br>
L - d[depth] excessive <br>
O - d[mtf1] excessive <br>
etc.</p>

<p style="margin-left:22%; margin-top: 1em">The NAV error
class has unique cases while VAL and GRAD classes are
described by alphabetic characters for each of the 24
numeric fields in MGD77 format order.</p>

<p style="margin-left:22%; margin-top: 1em">MGD77
bit-pattern w/ E77 alpha characters <br>

|-------------------------------------------------|----------|
<br>
| X W V U T S R Q P O N M L K J I H G F E D C B A | E77 Code
| <br>
| - - - - - - - - - - - - - - - - - - - - - - - - | - - - -
-| <br>
| n f e g m d m m m m b b d t p l l m h d m y t d | F I |
<br>
| q a o o s i s a t t t c e w t o a i o a o e z r | i D |
<br>
| c a t b d u e g f f c c p t c n t n u y n a t | e | <br>
| s r n 2 1 t r t r | l | <br>
| s h h | d | <br>
| - - - - - - - - - - - - - - - - - - - - - - - - | - - - -
-| <br>
| 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 | Bit
place| <br>
| - G C G C C - G G G - - G G - - - T T T T T - - | Bit type
| <br>

|-------------------------------------------------|----------|
<br>
Bit types: (G)eophysical, (C)orrection, (T)ime</p>

<h2>EXAMPLES
<a name="EXAMPLES"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em">To scan for
excessive values or gradients, try</p>


<p style="margin-left:11%; margin-top: 1em"><b>mgd77sniffer</b>
08010001</p>

<p style="margin-left:11%; margin-top: 1em">To dump cruise
gradients, try</p>


<p style="margin-left:11%; margin-top: 1em"><b>mgd77sniffer</b>
08010001 -Ds</p>

<p style="margin-left:11%; margin-top: 1em">To compare
cruise depth with ETOPO5 bathymetry and gravity with
Sandwell/Smith 2 min gravity version 11, try</p>


<p style="margin-left:11%; margin-top: 1em"><b>mgd77sniffer</b>
08010001 <b>&minus;G</b> depth,/data/GRIDS/etopo5_hdr.i2
<b>&minus;g</b> faa,/data/GRIDS/grav.11.2.img,0.1,1</p>

<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>



<p style="margin-left:11%; margin-top: 1em"><i><A HREF="mgd77list.html">mgd77list</A></i>(1),
<i><A HREF="mgd77track.html">mgd77track</A></i>(1) <i><A HREF="x2sys_init.html">x2sys_init</A></i>(1)</p>

<h2>REFERENCES
<a name="REFERENCES"></a>
</h2>


<p style="margin-left:11%; margin-top: 1em">Wessel, P., and
W. H. F. Smith, 2014, The Generic Mapping Tools (GMT)
version 4.5.12 Technical Reference &amp; Cookbook,
SOEST/NOAA. <br>
Wessel, P., and W. H. F. Smith, 1998, New, Improved Version
of Generic Mapping Tools Released, EOS Trans., AGU, 79 (47),
p. 579. <br>
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released, EOS Trans., AGU, 76 (33), p.
329. <br>
Wessel, P., and W. H. F. Smith, 1995, New Version of the
Generic Mapping Tools Released,
http://www.agu.org/eos_elec/95154e.html, Copyright 1995 by
the American Geophysical Union. <br>
Wessel, P., and W. H. F. Smith, 1991, Free Software Helps
Map and Display Data, EOS Trans., AGU, 72 (41), p. 441. <br>
The Marine Geophysical Data Exchange Format -
&quot;MGD77&quot;, see
<i>http://www.ngdc.noaa.gov/mgg/dat/geodas/docs/mgd77.txt</i></p>
<hr>
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