MAPPROJECT(1) Generic Mapping Tools MAPPROJECT(1)NAMEmapproject - Forward and Inverse map transformation of 2-D coordinates
SYNOPSISmapproject infiles -Jparameters -Rwest/east/south/north[r] [
-Ab|B|f|F[lon0/lat0] ] [ -C[dx/dy] ] [ -Dc|i|m|p ] [ -E[datum] ] [
-F[k|m|n|i|c|p] ] [ -G[x0/y0][+|-][/unit] ] [ -H[i][nrec] ] [ -I ] [
-Lline.xy[/unit][+] ] [ -Q[d|e ] [ -S ] [ -T[h]from[/to] ] [ -V ] [
-:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ] [
-g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] ] [ -m[i|o][flag] ]
DESCRIPTIONmapproject reads (longitude, latitude) positions from infiles [or stan‐
dard input] and computes (x,y) coordinates using the specified map pro‐
jection and scales. Optionally, it can read (x,y) positions and com‐
pute (longitude, latitude) values doing the inverse transformation.
This can be used to transform linear (x,y) points obtained by digitiz‐
ing a map of known projection to geographical coordinates. May also
calculate distances along track, to a fixed point, or closest approach
to a line. Finally, can be used to perform various datum conversions.
Additional data fields are permitted after the first 2 columns which
must have (longitude,latitude) or (x,y). See option -: on how to read
(latitude,longitude) files.
infiles
Data file(s) to be transformed. If not given, standard input is
read.
-J Selects the map projection. The following character determines
the projection. If the character is upper case then the argu‐
ment(s) supplied as scale(s) is interpreted to be the map width
(or axis lengths), else the scale argument(s) is the map scale
(see its definition for each projection). UNIT is cm, inch, or
m, depending on the MEASURE_UNIT setting in .gmtdefaults4, but
this can be overridden on the command line by appending c, i, or
m to the scale or width values. Append h, +, or - to the given
width if you instead want to set map height, the maximum dimen‐
sion, or the minimum dimension, respectively [Default is w for
width].
In case the central meridian is an optional parameter and it is
being omitted, then the center of the longitude range given by
the -R option is used. The default standard parallel is the
equator.
The ellipsoid used in the map projections is user-definable by
editing the .gmtdefaults4 file in your home directory. 73 com‐
monly used ellipsoids and spheroids are currently supported, and
users may also specify their own custum ellipsoid parameters
[Default is WGS-84]. Several GMT parameters can affect the pro‐
jection: ELLIPSOID, INTERPOLANT, MAP_SCALE_FACTOR, and MEA‐
SURE_UNIT; see the gmtdefaults man page for details.
Choose one of the following projections (The E or C after pro‐
jection names stands for Equal-Area and Conformal, respec‐
tively):
CYLINDRICAL PROJECTIONS:
-Jclon0/lat0/scale or -JClon0/lat0/width (Cassini).
Give projection center lon0/lat0 and scale (1:xxxx or
UNIT/degree).
-Jcyl_stere/[lon0/[lat0/]]scale or
-JCyl_stere/[lon0/[lat0/]]width (Cylindrical Stereographic).
Give central meridian lon0 (optional), standard parallel
lat0 (optional), and scale along parallel (1:xxxx or
UNIT/degree). The standard parallel is typically one of
these (but can be any value):
66.159467 - Miller's modified Gall
55 - Kamenetskiy's First
45 - Gall's Stereographic
30 - Bolshoi Sovietskii Atlas Mira or Kamenet‐
skiy's Second
0 - Braun's Cylindrical
-Jj[lon0/]scale or -JJ[lon0/]width (Miller Cylindrical Projec‐
tion).
Give the central meridian lon0 (optional) and scale
(1:xxxx or UNIT/degree).
-Jm[lon0/[lat0/]]scale or -JM[lon0/[lat0/]]width
Give central meridian lon0 (optional), standard parallel
lat0 (optional), and scale along parallel (1:xxxx or
UNIT/degree).
-Joparameters (Oblique Mercator [C]).
Typically used with -R<...>r, otherwise region is in
oblique coordinates. Specify one of:
-Jo[a]lon0/lat0/azimuth/scale or
-JO[a]lon0/lat0/azimuth/width
Set projection center lon0/lat0, azimuth of
oblique equator, and scale.
-Jo[b]lon0/lat0/lon1/lat1/scale or
-JO[b]lon0/lat0/lon1/lat1/scale
Set projection center lon0/lat0, another point on
the oblique equator lon1/lat1, and scale.
-Joclon0/lat0/lonp/latp/scale or
-JOclon0/lat0/lonp/latp/scale
Set projection center lon0/lat0, pole of oblique
projection lonp/latp, and scale.
Give scale along oblique equator (1:xxxx or UNIT/degree).
-Jq[lon0/[lat0/]]scale or -JQ[lon0/[lat0/]]width (Cylindrical
Equidistant).
Give the central meridian lon0 (optional), standard par‐
allel lat0 (optional), and scale (1:xxxx or UNIT/degree).
The standard parallel is typically one of these (but can
be any value):
61.7 - Grafarend and Niermann, minimum linear dis‐
tortion
50.5 - Ronald Miller Equirectangular
43.5 - Ronald Miller, minimum continental distor‐
tion
42 - Grafarend and Niermann
37.5 - Ronald Miller, minimum overall distortion
0 - Plate Carree, Simple Cylindrical, Plain/Plane
Chart
-Jtlon0/[lat0/]scale or -JTlon0/[lat0/]width
Give the central meridian lon0, central parallel lat0
(optional), and scale (1:xxxx or UNIT/degree).
-Juzone/scale or -JUzone/width (UTM - Universal Transverse Mer‐
cator [C]).
Give the UTM zone (A,B,1-60[C-X],Y,Z)) and scale (1:xxxx
or UNIT/degree).
Zones: If C-X not given, prepend - or + to enforce south‐
ern or northern hemisphere conventions [northern if south
> 0].
-Jy[lon0/[lat0/]]scale or -JY[lon0/[lat0/]]width (Cylindrical
Equal-Area [E]).
Give the central meridian lon0 (optional), standard par‐
allel lat0 (optional), and scale (1:xxxx or UNIT/degree).
The standard parallel is typically one of these (but can
be any value):
50 - Balthasart
45 - Gall-Peters
37.0666 - Caster
37.4 - Trystan Edwards
37.5 - Hobo-Dyer
30 - Behrman
0 - Lambert (default)
CONIC PROJECTIONS:
-Jblon0/lat0/lat1/lat2/scale or -JBlon0/lat0/lat1/lat2/width
(Albers [E]).
Give projection center lon0/lat0, two standard parallels
lat1/lat2, and scale (1:xxxx or UNIT/degree).
-Jdlon0/lat0/lat1/lat2/scale or -JDlon0/lat0/lat1/lat2/width
(Conic Equidistant)
Give projection center lon0/lat0, two standard parallels
lat1/lat2, and scale (1:xxxx or UNIT/degree).
-Jllon0/lat0/lat1/lat2/scale or -JLlon0/lat0/lat1/lat2/width
(Lambert [C])
Give origin lon0/lat0, two standard parallels lat1/lat2,
and scale along these (1:xxxx or UNIT/degree).
-Jpoly/[lon0/[lat0/]]scale or -JPoly/[lon0/[lat0/]]width ((Amer‐
ican) Polyconic).
Give the central meridian lon0 (optional), reference par‐
allel lat0 (optional, default = equator), and scale along
central meridian (1:xxxx or UNIT/degree).
AZIMUTHAL PROJECTIONS:
Except for polar aspects, -R w/e/s/n will be reset to -Rg. Use
-R<...>r for smaller regions.
-Jalon0/lat0[/horizon]/scale or -JAlon0/lat0[/horizon]/width
(Lambert [E]).
lon0/lat0 specifies the projection center. horizon spec‐
ifies the max distance from projection center (in
degrees, <= 180, default 90). Give scale as 1:xxxx or
radius/lat, where radius is distance in UNIT from origin
to the oblique latitude lat.
-Jelon0/lat0[/horizon]/scale or -JElon0/lat0[/horizon]/width
(Azimuthal Equidistant).
lon0/lat0 specifies the projection center. horizon spec‐
ifies the max distance from projection center (in
degrees, <= 180, default 180). Give scale as 1:xxxx or
radius/lat, where radius is distance in UNIT from origin
to the oblique latitude lat.
-Jflon0/lat0[/horizon]/scale or -JFlon0/lat0[/horizon]/width
(Gnomonic).
lon0/lat0 specifies the projection center. horizon spec‐
ifies the max distance from projection center (in
degrees, < 90, default 60). Give scale as 1:xxxx or
radius/lat, where radius is distance in UNIT from origin
to the oblique latitude lat.
-Jglon0/lat0[/horizon]/scale or -JGlon0/lat0[/horizon]/width
(Orthographic).
lon0/lat0 specifies the projection center. horizon spec‐
ifies the max distance from projection center (in
degrees, <= 90, default 90). Give scale as 1:xxxx or
radius/lat, where radius is distance in UNIT from origin
to the oblique latitude lat.
-Jglon0/lat0/altitude/azimuth/tilt/twist/Width/Height/scale or
-JGlon0/lat0/altitude/azimuth/tilt/twist/Width/Height/width
(General Perspective).
lon0/lat0 specifies the projection center. altitude is
the height (in km) of the viewpoint above local sea
level. If altitude is less than 10, then it is the dis‐
tance from the center of the earth to the viewpoint in
earth radii. If altitude has a suffix r then it is the
radius from the center of the earth in kilometers.
azimuth is measured to the east of north of view. tilt
is the upward tilt of the plane of projection. If tilt is
negative, then the viewpoint is centered on the horizon.
Further, specify the clockwise twist, Width, and Height
of the viewpoint in degrees. Give scale as 1:xxxx or
radius/lat, where radius is distance in UNIT from origin
to the oblique latitude lat.
-Jslon0/lat0[/horizon]/scale or -JSlon0/lat0[/horizon]/width
(General Stereographic [C]).
lon0/lat0 specifies the projection center. horizon spec‐
ifies the max distance from projection center (in
degrees, < 180, default 90). Give scale as 1:xxxx (true
at pole) or lat/1:xxxx (true at standard parallel lat) or
radius/lat (radius in UNIT from origin to the oblique
latitude lat). Note if 1:xxxx is used then to specify
horizon you must also specify the lat as +-90 to avoid
ambiguity.
MISCELLANEOUS PROJECTIONS:
-Jh[lon0/]scale or -JH[lon0/]width (Hammer [E]).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Ji[lon0/]scale or -JI[lon0/]width (Sinusoidal [E]).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jkf[lon0/]scale or -JKf[lon0/]width (Eckert IV) [E]).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jk[s][lon0/]scale or -JK[s][lon0/]width (Eckert VI) [E]).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jn[lon0/]scale or -JN[lon0/]width (Robinson).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jr[lon0/]scale -JR[lon0/]width (Winkel Tripel).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jv[lon0/]scale or -JV[lon0/]width (Van der Grinten).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
-Jw[lon0/]scale or -JW[lon0/]width (Mollweide [E]).
Give the central meridian lon0 (optional) and scale along
equator (1:xxxx or UNIT/degree).
NON-GEOGRAPHICAL PROJECTIONS:
-Jp[a]scale[/origin][r|z] or -JP[a]width[/origin][r|z] (Polar
coordinates (theta,r))
Optionally insert a after -Jp [ or -JP] for azimuths CW
from North instead of directions CCW from East [Default].
Optionally append /origin in degrees to indicate an angu‐
lar offset [0]). Finally, append r if r is elevations in
degrees (requires s >= 0 and n <= 90) or z if you want to
annotate depth rather than radius [Default]. Give scale
in UNIT/r-unit.
-Jxx-scale[/y-scale] or -JXwidth[/height] (Linear, log, and
power scaling)
Give x-scale (1:xxxx or UNIT/x-unit) and/or y-scale
(1:xxxx or UNIT/y-unit); or specify width and/or height
in UNIT. y-scale=x-scale if not specified separately and
using 1:xxxx implies that x-unit and y-unit are in
meters. Use negative scale(s) to reverse the direction
of an axis (e.g., to have y be positive down). Set height
or width to 0 to have it recomputed based on the implied
scale of the other axis. Optionally, append to x-scale,
y-scale, width or height one of the following:
d Data are geographical coordinates (in degrees).
l Take log10 of values before scaling.
ppower Raise values to power before scaling.
t Input coordinates are time relative to TIME_EPOCH.
T Input coordinates are absolute time.
Default axis lengths (see gmtdefaults) can be invoked
using -JXh (for landscape); -JXv (for portrait) will swap
the x- and y-axis lengths. The default unit for this
installation is either cm or inch, as defined in the file
share/gmt_setup.conf. However, you may change this by
editing your .gmtdefaults4 file(s).
-R xmin, xmax, ymin, and ymax specify the Region of interest. For
geographic regions, these limits correspond to west, east,
south, and north and you may specify them in decimal degrees or
in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
and upper right map coordinates are given instead of w/e/s/n.
The two shorthands -Rg and -Rd stand for global domain (0/360
and -180/+180 in longitude respectively, with -90/+90 in lati‐
tude). Alternatively, specify the name of an existing grid file
and the -R settings (and grid spacing, if applicable) are copied
from the grid. For calendar time coordinates you may either
give (a) relative time (relative to the selected TIME_EPOCH and
in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
time of the form [date]T[clock] (append T to -JX|x). At least
one of date and clock must be present; the T is always required.
The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
string must be of the form hh:mm:ss[.xxx]. The use of delim‐
iters and their type and positions must be exactly as indicated
(however, input, output and plot formats are customizable; see
gmtdefaults). Special case for the UTM projection: If -C is
used and -R is not given then the region is set to coincide with
the given UTM zone so as to preserve the full ellipsoidal solu‐
tion (See RESTRICTIONS for more information).
OPTIONS
No space between the option flag and the associated arguments.
infile(s)
input file(s) with 2 or more columns. If no file(s) is given,
mapproject will read the standard input.
-A[f|b]
-A calculates the (forward) azimuth from fixed point lon/lat to
each data point. Use -Ab to get back-azimuth from data points
to fixed point. Upper case F or B will convert from geodetic to
geocentric latitudes and estimate azimuth of geodesics (assuming
the current ellipsoid is not a sphere). If no fixed point is
given then we compute the azimuth (or back-azimuth) from the
previous point.
-C Set center of projected coordinates to be at map projection cen‐
ter [Default is lower left corner]. Optionally, add offsets in
the projected units to be added (or subtracted when -I is set)
to (from) the projected coordinates, such as false eastings and
northings for particular projection zones [0/0]. The unit used
for the offsets is the plot distance unit in effect (see MEA‐
SURE_UNIT) unless -F is used, in which case the offsets are
always in meters.
-D Temporarily override MEASURE_UNIT and use c (cm), i (inch), m
(meter), or p (points) instead. Cannot be used with -F.
-E Convert from geodetic (lon, lat, height) to Earth Centered Earth
Fixed (ECEF) (x,y,z) coordinates (add -I for the inverse conver‐
sion). Append datum ID (see -Qd) or give ellipsoid:dx,dy,dz
where ellipsoid may be an ellipsoid ID (see -Qe) or given as
a[,inv_f], where a is the semi-major axis and inv_f is the
inverse flattening (0 if omitted). If datum is - or not given
we assume WGS-84.
-F Force 1:1 scaling, i.e., output (or input, see -I) data are in
actual projected meters. To specify other units, append k (km),
m (mile), n (nautical mile), i (inch), c (cm), or p (points).
Without -F, the output (or input, see -I) are in the units spec‐
ified by MEASURE_UNIT (but see -D).
-G Calculate distances along track OR to the optional point set
with -Gx0/y0. Append IT(unit), the distance unit; choose among
e (m), k (km), m (mile), n (nautical mile), d (spherical
degree), c (Cartesian distance using input coordinates) or C
(Cartesian distance using projected coordinates). The last unit
requires -R and -J to be set. Upper case E, K, M, N, or D will
use exact methods for geodesic distances (Rudoe's method for
distances in length units and employing geocentric latitudes in
degree calculations, assuming the current ellipsoid is not
spherical). With no fixed point we calculate cumulate distances
along track. To obtain incremental distance between successive
points, use -G-. To specify the 2nd point via two extra columns
in the input file, choose -G+.
-H Input file(s) has header record(s). If used, the default number
of header records is N_HEADER_RECS. Use -Hi if only input data
should have header records [Default will write out header
records if the input data have them]. Blank lines and lines
starting with # are always skipped.
-I Do the Inverse transformation, i.e., get (longitude,latitude)
from (x,y) data.
-L Determine the shortest distance from the input data points to
the line(s) given in the ASCII multi-segment file line.xy. The
distance and the coordinates of the nearest point will be
appended to the output as three new columns. Append the dis‐
tance unit; choose among e (m), k (km), m (mile), n (nautical
mile), d (spherical degree), c (Cartesian distance using input
coordinates) or C (Cartesian distance using projected coordi‐
nates). The last unit requires -R and -J to be set. A spheri‐
cal approximation is used for geographic data. Finally, append
+ to report the line segment id and the fractional point number
instead of lon/lat of the nearest point.
-Q List all projection parameters. To only list datums, use -Qd.
To only list ellipsoids, use -Qe.
-S Suppress points that fall outside the region.
-T Coordinate conversions between datums from and to using the
standard Molodensky transformation. Use -Th if 3rd input column
has height above ellipsoid [Default assumes height = 0, i.e., on
the ellipsoid]. Specify datums using the datum ID (see -Qd) or
give ellipsoid:dx,dy,dz where ellipsoid may be an ellipsoid ID
(see -Qe) or given as a[,inv_f], where a is the semi-major axis
and inv_f is the inverse flattening (0 if omitted). If datum is
- or not given we assume WGS-84. -T may be used in conjunction
with -R -J to change the datum before coordinate projection (add
-I to apply the datum conversion after the inverse projection).
Make sure that the ELLIPSOID setting is correct for your case.
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-: Toggles between (longitude,latitude) and (latitude,longitude)
input and/or output. [Default is (longitude,latitude)]. Append
i to select input only or o to select output only. [Default
affects both].
-bi Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of columns in your binary
input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1/var2/... to specify the variables to be read. [Default is
2 input columns].
-bo Selects binary output. Append s for single precision [Default
is d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of desired columns in your
binary output file. [Default is same as input].
-f Special formatting of input and/or output columns (time or geo‐
graphical data). Specify i or o to make this apply only to
input or output [Default applies to both]. Give one or more
columns (or column ranges) separated by commas. Append T (abso‐
lute calendar time), t (relative time in chosen TIME_UNIT since
TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
to each column or column range item. Shorthand -f[i|o]g means
-f[i|o]0x,1y (geographic coordinates).
-g Examine the spacing between consecutive data points in order to
impose breaks in the line. Append x|X or y|Y to define a gap
when there is a large enough change in the x or y coordinates,
respectively, or d|D for distance gaps; use upper case to calcu‐
late gaps from projected coordinates. For gap-testing on other
columns use [col]z; if col is not prepended the it defaults to 2
(i.e., 3rd column). Append [+|-]gap and optionally a unit u.
Regarding optional signs: -ve means previous minus current col‐
umn value must exceed |gap to be a gap, +ve means current minus
previous column value must exceed gap, and no sign means the
absolute value of the difference must exceed gap. For geo‐
graphic data (x|y|d), the unit u may be meter [Default], kilome‐
ter, miles, or nautical miles. For projected data (X|Y|D),
choose from inch, centimeter, meter, or points [Default unit set
by MEASURE_UNIT]. Note: For x|y|z with time data the unit is
instead controlled by TIME_UNIT. Repeat the option to specify
multiple criteria, of which any can be met to produce a line
break. Issue an additional -ga to indicate that all criteria
must be met instead.
-m Multiple segment file(s). Segments are separated by a special
record. For ASCII files the first character must be flag
[Default is '>']. For binary files all fields must be NaN and
-b must set the number of output columns explicitly. By default
the -m setting applies to both input and output. Use -mi and
-mo to give separate settings to input and output.
ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters
in your .gmtdefaults4 file. Longitude and latitude are formatted
according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
according to D_FORMAT. Be aware that the format in effect can lead to
loss of precision in the output, which can lead to various problems
downstream. If you find the output is not written with enough preci‐
sion, consider switching to binary output (-bo if available) or specify
more decimals using the D_FORMAT setting.
EXAMPLES
To transform a file with (longitude,latitude) into (x,y) positions in
cm on a Mercator grid for a given scale of 0.5 cm per degree, run
mapproject lonlatfile -R 20/50/12/25 -Jm 0.5c > xyfile
To transform several 2-column, binary, double precision files with
(latitude,longitude) into (x,y) positions in inch on a Transverse Mer‐
cator grid (central longitude 75W) for scale = 1:500000 and suppress
those points that would fall outside the map area, run
mapproject tracks.* -R-80/-70/20/40 -Jt-75/1:500000 -: -S -Di -bo -bi 2
> tmfile.b
To convert the geodetic coordinates (lon, lat, height) in the file
old.dat from the NAD27 CONUS datum (Datum ID 131 which uses the
Clarke-1866 ellipsoid) to WGS 84, run
mapproject old.dat -Th 131 > new.dat
To compute the closest distance (in km) between each point in the input
file quakes.dat and the line segments given in the multi-segment ASCII
file coastline.xy, run
mapproject quakes.dat -L coastline.xy/k > quake_dist.dat
RESTRICTIONS
The rectangular input region set with -R will in general be mapped into
a non-rectangular grid. Unless -C is set, the leftmost point on this
grid has xvalue = 0.0, and the lowermost point will have yvalue = 0.0.
Thus, before you digitize a map, run the extreme map coordinates
through mapproject using the appropriate scale and see what (x,y) val‐
ues they are mapped onto. Use these values when setting up for digi‐
tizing in order to have the inverse transformation work correctly, or
alternatively, use awk to scale and shift the (x,y) values before
transforming.
For some projections, a spherical solution may be used despite the user
having selected an ellipsoid. This occurs when the users -R setting
implies a region that exceeds the domain in which the ellipsoidal
series expansions are valid. These are the conditions: (1) Lambert
Conformal Conic (-JL) and Albers Equal-Area (-JB) will use the spheri‐
cal solution when the map scale exceeds 1.0E7. (2) Transverse Mercator
(-JT) and UTM (-JU) will will use the spherical solution when either
the west or east boundary given in -R is more than 10 degrees from the
central meridian, and (3) same for Cassini (-JC) but with a limit of
only 4 degrees.
ELLIPSOIDS AND SPHEROIDS
GMT will use ellipsoidal formulae if they are implemented and the user
have selected an ellipsoid as the reference shape (see ELLIPSOID in
gmtdefaults). The user needs to be aware of a few potential pitfalls:
(1) For some projections, such as Transverse Mercator, Albers, and
Lamberts conformal conic we use the ellipsoidal expressions when the
areas mapped are small, and switch to the spherical expressions (and
substituting the appropriate auxiliary latitudes) for larger maps. The
ellipsoidal formulae are used as follows: (a) Transverse Mercator: When
all points are within 10 degrees of central meridian, (b) Conic projec‐
tions when longitudinal range is less than 90 degrees, (c) Cassini pro‐
jection when all points are within 4 degrees of central meridian. (2)
When you are trying to match some historical data (e.g., coordinates
obtained with a certain projection and a certain reference ellipsoid)
you may find that GMT gives results that are slightly different. One
likely source of this mismatch is that older calculations often used
less significant digits. For instance, Snyder's examples often use the
Clarke 1866 ellipsoid (defined by him as having a flattening f =
1/294.98). From f we get the eccentricity squared to be 0.00676862818
(this is what GMT uses), while Snyder rounds off and uses 0.00676866.
This difference can give discrepancies of several tens of cm. If you
need to reproduce coordinates projected with this slightly different
eccentricity, you should specify your own ellipsoid with the same
parameters as Clarke 1866, but with f = 1/294.97861076. Also, be aware
that older data may be referenced to different datums, and unless you
know which datum was used and convert all data to a common datum you
may experience mismatches of tens to hundreds of meters. (3) Finally,
be aware that MAP_SCALE_FACTOR have certain default values for some
projections so you may have to override the setting in order to match
results produced with other settings.
SEE ALSOgmtdefaults(1), GMT(1), project(1)REFERENCES
Bomford, G., 1952, Geodesy, Oxford U. Press.
Snyder, J. P., 1987, Map Projections - A Working Manual, U.S. Geologi‐
cal Survey Prof. Paper 1395.
Vanicek, P. and Krakiwsky, E, 1982, Geodesy - The Concepts, North-Hol‐
land Publ., ISBN: 0 444 86149 1.
GMT 4.5.14 1 Nov 2015 MAPPROJECT(1)
