mapproject
mapproject - Forward and Inverse map transformation of 2-D
coordinates
SYNOPSIS
mapproject infiles -Jparameters -Rwest/east/south/north[r]
[ -C ] [ -Dc|i|m|p ] [ -F[k|m|n|i|c|p] ] [ -H[nrec] ] [ -I
] [ -M[flag] ] [ -S ] [ -V ] [ -: ] [ -bi[s][n] ] [ -bo[s]
]
DESCRIPTION
mapproject reads (longitude, latitude) positions from
infiles [or standard input] and computes (x,y) coordinates
using the specified map projection and scales. Option
ally, it can read (x,y) positions and compute (longitude,
latitude) values doing the inverse transformation. This
can be used to transform linear (x,y) points obtained by
digitizing a map of known projection to geographical coor
dinates. 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.
No space between the option flag and the associ
ated arguments. Use upper case for the option flags and
lower case for modifiers.
infiles
Data file(s) to be transformed. If not given, stan
dard input is read.
-J Selects the map projection. The following character
determines the projection. If the character is
upper case then the argument(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 MEA
SURE_UNIT setting in .gmtdefaults, but this can be
overridden on the command line by appending c, i,
or m to the scale/width values. Choose one of the
following projections (The E or C after projection
names stands for Equal-Area and Conformal, respec
tively):
CYLINDRICAL PROJECTIONS:
-Jclon0/lat0/scale or -JClon0/lat0/width (Cassini)
Give projection center and scale (1:xxxx or
UNIT/degree).
-Jjlon0/scale or -JJlon0/width (Miller Cylindrical
Projection)
Give the central meridian and scale (1:xxxx
or UNIT/degree).
Give scale along equator (1:xxxx or
UNIT/degree).
-Jmlon0/lat0/scale or -JMlon0/lat0/width
Give central meridian, standard
latitude and scale along parallel (1:xxxx or
UNIT/degree).
-Joparameters (Oblique Mercator [C]). Specify one
of:
-Joalon0/lat0/azimuth/scale or
-JOalon0/lat0/azimuth/width
Set projection center, azimuth of
oblique equator, and scale.
-Joblon0/lat0/lon1/lat1/scale or
-JOblon0/lat0/lon1/lat1/scale
Set projection center, another
point on the oblique equator, and scale.
-Joclon0/lat0/lonp/latp/scale or
-JOclon0/lat0/lonp/latp/scale
Set projection center, pole of
oblique projection, and scale.
Give scale along oblique equator (1:xxxx or
UNIT/degree).
-Jqlon0/scale or -JQlon0/width (Equidistant Cylin
drical Projection (Plate Carree))
Give the central meridian and scale (1:xxxx
or UNIT/degree).
-Jtparameters (Transverse Mercator [C]). Specify
one of:
-Jtlon0/scale or -JTlon0/width
Give the central meridian and scale
(1:xxxx or UNIT/degree).
-Jtlon0/lat0/scale or -JTlon0/lat0/width
Give projection center and scale
(1:xxxx or UNIT/degree).
-Juzone/scale or -JUzone/width (UTM - Universal
Transverse Mercator [C])
Give the zone number (1-60) and scale
(1:xxxx or UNIT/degree).
zones: prepend - or + to enforce southern
or northern hemisphere conventions [northern if
south > 0].
-Jylon0/lats/scale or -JYlon0/lats/width (Basic
Cylindrical Projections [E])
Give the central meridian, standard paral
lel, and scale (1:xxxx or UNIT/degree).
The standard parallel is typically one of
these (but can be any value):
45 - The Peters projection
37.4 - The Trystan Edwards projection
30 - The Behrman projection
0 - The Lambert projection
[E]).
lon0/lat0 specifies the projection center.
Give scale as 1:xxxx or radius/lat, where
radius is distance
in UNIT from origin to the oblique latitude
lat.
-Jelon0/lat0/scale or -JElon0/lat0/width (Equidis
tant).
lon0/lat0 specifies the projection center.
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/hori
zon/width (Gnomonic).
lon0/lat0 specifies the projection center.
horizon specifies the max distance from
projection center (in degrees, < 90).
Give scale as 1:xxxx or radius/lat, where
radius is distance
in UNIT from origin to the oblique latitude
lat.
-Jglon0/lat0/scale or -JGlon0/lat0/width (Ortho
graphic).
lon0/lat0 specifies the projection center.
Give scale as 1:xxxx or radius/lat, where
radius is distance
in UNIT from origin to the oblique latitude
lat.
-Jslon0/lat0/scale or -JSlon0/lat0/width (General
Stereographic [C])
lon0/lat0 specifies the projection center.
Give scale as 1:xxxx (true at pole) or
slat/1:xxxx (true at standard parallel slat)
or radius/lat (radius in UNIT from origin
to the oblique latitude lat).
CONIC PROJECTIONS:
-Jblon0/lat0/lat1/lat2/scale or
-JBlon0/lat0/lat1/lat2/width (Albers [E])
Give projection center, two standard paral
lels, and scale (1:xxxx or UNIT/degree).
-Jdlon0/lat0/lat1/lat2/scale or
-JDlon0/lat0/lat1/lat2/width (Equidistant)
Give projection center, two standard paral
lels, and scale (1:xxxx or UNIT/degree).
-Jllon0/lat0/lat1/lat2/scale or
-JLlon0/lat0/lat1/lat2/width (Lambert [C])
Give origin, 2 standard parallels, and
scale along these (1:xxxx or UNIT/degree).
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jilon0/scale or -JIlon0/width (Sinusoidal [E])
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jk[f|s]lon0/scale or -JK[f|s]lon0/width (Eckert IV
(f) and VI (s) [E])
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jnlon0/scale or -JNlon0/width (Robinson)
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jrlon0/scale -JRlon0/width (Winkel Tripel)
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jvlon0/scale or -JVlon0/width (Van der Grinten)
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
-Jwlon0/scale or -JWlon0/width (Mollweide [E])
Give the central meridian and scale along
equator (1:xxxx or UNIT/degree).
NON-GEOGRAPHICAL PROJECTIONS:
-Jp[a]scale[/origin] or -JP[a]width[/origin] (Lin
ear projection for polar (theta,r) coordinates,
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 angular offset [0]).
Give scale in UNIT/r-unit.
-Jxx-scale[/y-scale] or -JXwidth[/height]
scale [or width] can be any of the following 3
types:
-Jxscale - Regular linear scaling.
-Jxscalel - Take log10 of values before
scaling.
-Jxscaleppower - Raise values to power
before scaling.
Give x-scale in UNIT/x-unit and y-scale in UNIT/y-
unit. (y-scale = x-scale if not specified sepa
rately). Use negative scale(s) to reverse the
direction of an axis (e.g., to have y be positive
down).
Append a single d if data are geographical coordi
nates in degrees. Default axes lengths (see gmtde
faults) can be invoked using -JXh (for landscape);
-JXv (for portrait) will swap the x- and y-axes
lengths. The GMT default unit for this installa
tion is UNIT. However, you may change this by edit
ing your .gmtdefaults file(s) (run gmtdefaults to
is user-definable by editing the .gmtdefaults file
in your home directory. 13 commonly used ellipsoids
and a spheroid are currently supported, and users
may also specify their own ellipsoid parameters
(see man gmtdefaults for more details). GMT
default is WGS-84. Several GMT parameters can
affect the projection: ELLIPSOID, INTERPOLANT,
MAP_SCALE_FACTOR, and MEASURE_UNIT; see the gmtde
faults man page for details.
-R west, east, south, and north specify the Region of
interest. To specify boundaries in degrees and min
utes [and seconds], use the dd:mm[:ss] format.
Append r if lower left and upper right map coordi
nates are given instead of wesn.
OPTIONS
infile(s)
input file(s) with 2 or more columns. If no file(s)
is given, mapproject will read standard input.
-C Set center of projected coordinates to be at map
projection center [Default is lower left corner].
-D Temporarily override MEASURE_UNIT and use c (cm), i
(inch), m (meter), or p (points) instead. Cannot be
used with -F.
-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
specified by MEASURE_UNIT (but see -D).
-H Input file(s) has Header record(s). Number of
header records can be changed by editing your .gmt
defaults file. If used, GMT default is 1 header
record.
-I Do the Inverse transformation, i.e. get (longi
tude,latitude) from (x,y) data.
-M Multiple segment file(s). Segments are separated by
a special record. For ASCII files the first char
acter must be flag [Default is '>']. For binary
files all fields must be NaN.
-S Suppress points that fall outside the region.
-V Selects verbose mode, which will send progress
reports to stderr [Default runs "silently"].
tude,longitude) input/output. [Default is (longi
tude,latitude)]. Applies to geographic coordinates
only.
-bi Selects binary input. Append s for single precision
[Default is double]. Append n for the number of
columns in the binary file(s). [Default is 2 input
columns]
-bo Selects binary output. Append s for single preci
sion [Default is double].
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 -R20/50/12/25 -Jm0.5c > xyfile
To transform several 2-column, binary, double precision
files with (latitude,longitude) into (x,y) positions in
inch on a Transverse Mercator 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 -bi2 > tmfile.b
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)
values they are mapped onto. Use these values when setting
up for digitizing in order to have the inverse transforma
tion work correctly, or alternatively, use awk to scale
and shift the (x,y) values before transforming.
ELLIPSOIDS AND SPHEROIDS
GMT will use ellipsoidal formulae if they are implemented
and the user have selected an ellipsoid as the reference
shape (see 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 auxillary latitudes) for
larger maps. The ellipsoidal formulae are used are fol
lows: (a) Transverse Mercator: When all points are within
10 degrees of central meridian, (b) Conic projections when
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 exam
ples 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 10 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.
SEE ALSO
gmtdefaults(l), gmt(l), project(l)
REFERENCES
Snyder, J. P., 1987, Map Projections - A Working Manual,
U.S. Geological Survey Prof. Paper 1395.
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