1MAPPROJECT(1) Generic Mapping Tools MAPPROJECT(1)
2
6 mapproject - Forward and Inverse map transformation of 2-D coordinates
7
9 mapproject infiles -Jparameters -Rwest/east/south/north[r] [
10 -Ab|B|f|F[lon0/lat0] ] [ -C[dx/dy] ] [ -Dc|i|m|p ] [ -E[datum] ] [
11 -F[k|m|n|i|c|p] ] [ -G[x0/y0][+|-][/unit] ] [ -H[i][nrec] ] [ -I ] [
12 -Lline.xy[/unit][+] ] [ -Q[d|e ] [ -S ] [ -T[h]from[/to] ] [ -V ] [
13 -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ] [
14 -g[a]x|y|d|X|Y|D|[col]z[+|-]gap[u] ] [ -m[i|o][flag] ]
15
17 mapproject reads (longitude, latitude) positions from infiles [or stan‐
18 dard input] and computes (x,y) coordinates using the specified map pro‐
19 jection and scales. Optionally, it can read (x,y) positions and com‐
20 pute (longitude, latitude) values doing the inverse transformation.
21 This can be used to transform linear (x,y) points obtained by digitiz‐
22 ing a map of known projection to geographical coordinates. May also
23 calculate distances along track, to a fixed point, or closest approach
24 to a line. Finally, can be used to perform various datum conversions.
25 Additional data fields are permitted after the first 2 columns which
26 must have (longitude,latitude) or (x,y). See option -: on how to read
27 (latitude,longitude) files.
28 infiles
30 Data file(s) to be transformed. If not given, standard input is
31 read.
32 -J Selects the map projection. The following character determines
34 the projection. If the character is upper case then the argu‐
35 ment(s) supplied as scale(s) is interpreted to be the map width
36 (or axis lengths), else the scale argument(s) is the map scale
37 (see its definition for each projection). UNIT is cm, inch, or
38 m, depending on the MEASURE_UNIT setting in .gmtdefaults4, but
39 this can be overridden on the command line by appending c, i, or
40 m to the scale or width values. Append h, +, or - to the given
41 width if you instead want to set map height, the maximum dimen‐
42 sion, or the minimum dimension, respectively [Default is w for
43 width].
44 In case the central meridian is an optional parameter and it is
45 being omitted, then the center of the longitude range given by
46 the -R option is used. The default standard parallel is the
47 equator.
48 The ellipsoid used in the map projections is user-definable by
49 editing the .gmtdefaults4 file in your home directory. 73 com‐
50 monly used ellipsoids and spheroids are currently supported, and
51 users may also specify their own custum ellipsoid parameters
52 [Default is WGS-84]. Several GMT parameters can affect the pro‐
53 jection: ELLIPSOID, INTERPOLANT, MAP_SCALE_FACTOR, and MEA‐
54 SURE_UNIT; see the gmtdefaults man page for details.
55 Choose one of the following projections (The E or C after pro‐
56 jection names stands for Equal-Area and Conformal, respec‐
57 tively):
58 CYLINDRICAL PROJECTIONS:
60 -Jclon0/lat0/scale or -JClon0/lat0/width (Cassini).
62 Give projection center lon0/lat0 and scale (1:xxxx or
63 UNIT/degree).
64 -Jcyl_stere/[lon0/[lat0/]]scale or
66 -JCyl_stere/[lon0/[lat0/]]width (Cylindrical Stereographic).
67 Give central meridian lon0 (optional), standard parallel
68 lat0 (optional), and scale along parallel (1:xxxx or
69 UNIT/degree). The standard parallel is typically one of
70 these (but can be any value):
71 66.159467 - Miller's modified Gall
72 55 - Kamenetskiy's First
73 45 - Gall's Stereographic
74 30 - Bolshoi Sovietskii Atlas Mira or Kamenet‐
75 skiy's Second
76 0 - Braun's Cylindrical
77 -Jj[lon0/]scale or -JJ[lon0/]width (Miller Cylindrical Projec‐
79 tion).
80 Give the central meridian lon0 (optional) and scale
81 (1:xxxx or UNIT/degree).
82 -Jm[lon0/[lat0/]]scale or -JM[lon0/[lat0/]]width
84 Give central meridian lon0 (optional), standard parallel
85 lat0 (optional), and scale along parallel (1:xxxx or
86 UNIT/degree).
87 -Joparameters (Oblique Mercator [C]).
89 Specify one of:
90 -Jo[a]lon0/lat0/azimuth/scale or
92 -JO[a]lon0/lat0/azimuth/width
93 Set projection center lon0/lat0, azimuth of
94 oblique equator, and scale.
95 -Jo[b]lon0/lat0/lon1/lat1/scale or
97 -JO[b]lon0/lat0/lon1/lat1/scale
98 Set projection center lon0/lat0, another point on
99 the oblique equator lon1/lat1, and scale.
100 -Joclon0/lat0/lonp/latp/scale or
102 -JOclon0/lat0/lonp/latp/scale
103 Set projection center lon0/lat0, pole of oblique
104 projection lonp/latp, and scale.
105 Give scale along oblique equator (1:xxxx or UNIT/degree).
107 -Jq[lon0/[lat0/]]scale or -JQ[lon0/[lat0/]]width (Cylindrical
109 Equidistant).
110 Give the central meridian lon0 (optional), standard par‐
111 allel lat0 (optional), and scale (1:xxxx or UNIT/degree).
112 The standard parallel is typically one of these (but can
113 be any value):
114 61.7 - Grafarend and Niermann, minimum linear dis‐
115 tortion
116 50.5 - Ronald Miller Equirectangular
117 43.5 - Ronald Miller, minimum continental distor‐
118 tion
119 42 - Grafarend and Niermann
120 37.5 - Ronald Miller, minimum overall distortion
121 0 - Plate Carree, Simple Cylindrical, Plain/Plane
122 Chart
123 -Jtlon0/[lat0/]scale or -JTlon0/[lat0/]width
125 Give the central meridian lon0, central parallel lat0
126 (optional), and scale (1:xxxx or UNIT/degree).
127 -Juzone/scale or -JUzone/width (UTM - Universal Transverse Mer‐
129 cator [C]).
130 Give the UTM zone (A,B,1-60[C-X],Y,Z)) and scale (1:xxxx
131 or UNIT/degree).
132 Zones: If C-X not given, prepend - or + to enforce south‐
133 ern or northern hemisphere conventions [northern if south
134 > 0].
135 -Jy[lon0/[lat0/]]scale or -JY[lon0/[lat0/]]width (Cylindrical
137 Equal-Area [E]).
138 Give the central meridian lon0 (optional), standard par‐
139 allel lat0 (optional), and scale (1:xxxx or UNIT/degree).
140 The standard parallel is typically one of these (but can
141 be any value):
142 50 - Balthasart
143 45 - Gall-Peters
144 37.0666 - Caster
145 37.4 - Trystan Edwards
146 37.5 - Hobo-Dyer
147 30 - Behrman
148 0 - Lambert (default)
149 CONIC PROJECTIONS:
151 -Jblon0/lat0/lat1/lat2/scale or -JBlon0/lat0/lat1/lat2/width
153 (Albers [E]).
154 Give projection center lon0/lat0, two standard parallels
155 lat1/lat2, and scale (1:xxxx or UNIT/degree).
156 -Jdlon0/lat0/lat1/lat2/scale or -JDlon0/lat0/lat1/lat2/width
158 (Conic Equidistant)
159 Give projection center lon0/lat0, two standard parallels
160 lat1/lat2, and scale (1:xxxx or UNIT/degree).
161 -Jllon0/lat0/lat1/lat2/scale or -JLlon0/lat0/lat1/lat2/width
163 (Lambert [C])
164 Give origin lon0/lat0, two standard parallels lat1/lat2,
165 and scale along these (1:xxxx or UNIT/degree).
166 -Jpoly/[lon0/[lat0/]]scale or -JPoly/[lon0/[lat0/]]width ((Amer‐
168 ican) Polyconic).
169 Give the central meridian lon0 (optional), reference par‐
170 allel lat0 (optional, default = equator), and scale along
171 central meridian (1:xxxx or UNIT/degree).
172 AZIMUTHAL PROJECTIONS:
174 Except for polar aspects, -Rw/e/s/n will be reset to -Rg. Use
176 -R<...>r for smaller regions.
177 -Jalon0/lat0[/horizon]/scale or -JAlon0/lat0[/horizon]/width
179 (Lambert [E]).
180 lon0/lat0 specifies the projection center. horizon spec‐
181 ifies the max distance from projection center (in
182 degrees, <= 180, default 90). Give scale as 1:xxxx or
183 radius/lat, where radius is distance in UNIT from origin
184 to the oblique latitude lat.
185 -Jelon0/lat0[/horizon]/scale or -JElon0/lat0[/horizon]/width
187 (Azimuthal Equidistant).
188 lon0/lat0 specifies the projection center. horizon spec‐
189 ifies the max distance from projection center (in
190 degrees, <= 180, default 180). Give scale as 1:xxxx or
191 radius/lat, where radius is distance in UNIT from origin
192 to the oblique latitude lat.
193 -Jflon0/lat0[/horizon]/scale or -JFlon0/lat0[/horizon]/width
195 (Gnomonic).
196 lon0/lat0 specifies the projection center. horizon spec‐
197 ifies the max distance from projection center (in
198 degrees, < 90, default 60). Give scale as 1:xxxx or
199 radius/lat, where radius is distance in UNIT from origin
200 to the oblique latitude lat.
201 -Jglon0/lat0[/horizon]/scale or -JGlon0/lat0[/horizon]/width
203 (Orthographic).
204 lon0/lat0 specifies the projection center. horizon spec‐
205 ifies the max distance from projection center (in
206 degrees, <= 90, default 90). Give scale as 1:xxxx or
207 radius/lat, where radius is distance in UNIT from origin
208 to the oblique latitude lat.
209 -Jglon0/lat0/altitude/azimuth/tilt/twist/Width/Height/scale or
211 -JGlon0/lat0/altitude/azimuth/tilt/twist/Width/Height/width
212 (General Perspective).
213 lon0/lat0 specifies the projection center. altitude is
214 the height (in km) of the viewpoint above local sea
215 level. If altitude is less than 10, then it is the dis‐
216 tance from the center of the earth to the viewpoint in
217 earth radii. If altitude has a suffix r then it is the
218 radius from the center of the earth in kilometers.
219 azimuth is measured to the east of north of view. tilt
220 is the upward tilt of the plane of projection. If tilt is
221 negative, then the viewpoint is centered on the horizon.
222 Further, specify the clockwise twist, Width, and Height
223 of the viewpoint in degrees. Give scale as 1:xxxx or
224 radius/lat, where radius is distance in UNIT from origin
225 to the oblique latitude lat.
226 -Jslon0/lat0[/horizon]/scale or -JSlon0/lat0[/horizon]/width
228 (General Stereographic [C]).
229 lon0/lat0 specifies the projection center. horizon spec‐
230 ifies the max distance from projection center (in
231 degrees, < 180, default 90). Give scale as 1:xxxx (true
232 at pole) or lat0/1:xxxx (true at standard parallel lat0)
233 or radius/lat (radius in UNIT from origin to the oblique
234 latitude lat). Note if 1:xxxx is used then to specify
235 horizon you must also specify the lat0 as +-90 to avoid
236 ambiguity.
237 MISCELLANEOUS PROJECTIONS:
239 -Jh[lon0/]scale or -JH[lon0/]width (Hammer [E]).
241 Give the central meridian lon0 (optional) and scale along
242 equator (1:xxxx or UNIT/degree).
243 -Ji[lon0/]scale or -JI[lon0/]width (Sinusoidal [E]).
245 Give the central meridian lon0 (optional) and scale along
246 equator (1:xxxx or UNIT/degree).
247 -Jkf[lon0/]scale or -JKf[lon0/]width (Eckert IV) [E]).
249 Give the central meridian lon0 (optional) and scale along
250 equator (1:xxxx or UNIT/degree).
251 -Jk[s][lon0/]scale or -JK[s][lon0/]width (Eckert VI) [E]).
253 Give the central meridian lon0 (optional) and scale along
254 equator (1:xxxx or UNIT/degree).
255 -Jn[lon0/]scale or -JN[lon0/]width (Robinson).
257 Give the central meridian lon0 (optional) and scale along
258 equator (1:xxxx or UNIT/degree).
259 -Jr[lon0/]scale -JR[lon0/]width (Winkel Tripel).
261 Give the central meridian lon0 (optional) and scale along
262 equator (1:xxxx or UNIT/degree).
263 -Jv[lon0/]scale or -JV[lon0/]width (Van der Grinten).
265 Give the central meridian lon0 (optional) and scale along
266 equator (1:xxxx or UNIT/degree).
267 -Jw[lon0/]scale or -JW[lon0/]width (Mollweide [E]).
269 Give the central meridian lon0 (optional) and scale along
270 equator (1:xxxx or UNIT/degree).
271 NON-GEOGRAPHICAL PROJECTIONS:
273 -Jp[a]scale[/origin][r|z] or -JP[a]width[/origin][r|z] (Polar
275 coordinates (theta,r))
276 Optionally insert a after -Jp [ or -JP] for azimuths CW
277 from North instead of directions CCW from East [Default].
278 Optionally append /origin in degrees to indicate an angu‐
279 lar offset [0]). Finally, append r if r is elevations in
280 degrees (requires s >= 0 and n <= 90) or z if you want to
281 annotate depth rather than radius [Default]. Give scale
282 in UNIT/r-unit.
283 -Jxx-scale[/y-scale] or -JXwidth[/height] (Linear, log, and
285 power scaling)
286 Give x-scale (1:xxxx or UNIT/x-unit) and/or y-scale
287 (1:xxxx or UNIT/y-unit); or specify width and/or height
288 in UNIT. y-scale=x-scale if not specified separately and
289 using 1:xxxx implies that x-unit and y-unit are in
290 meters. Use negative scale(s) to reverse the direction
291 of an axis (e.g., to have y be positive down). Set height
292 or width to 0 to have it recomputed based on the implied
293 scale of the other axis. Optionally, append to x-scale,
294 y-scale, width or height one of the following:
295 d Data are geographical coordinates (in degrees).
297 l Take log10 of values before scaling.
299 ppower Raise values to power before scaling.
301 t Input coordinates are time relative to TIME_EPOCH.
303 T Input coordinates are absolute time.
305 Default axis lengths (see gmtdefaults) can be invoked
307 using -JXh (for landscape); -JXv (for portrait) will swap
308 the x- and y-axis lengths. The default unit for this
309 installation is either cm or inch, as defined in the file
310 share/gmt.conf. However, you may change this by editing
311 your .gmtdefaults4 file(s).
312 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
314 geographic regions, these limits correspond to west, east,
315 south, and north and you may specify them in decimal degrees or
316 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
317 and upper right map coordinates are given instead of w/e/s/n.
318 The two shorthands -Rg and -Rd stand for global domain (0/360
319 and -180/+180 in longitude respectively, with -90/+90 in lati‐
320 tude). Alternatively, specify the name of an existing grid file
321 and the -R settings (and grid spacing, if applicable) are copied
322 from the grid. For calendar time coordinates you may either
323 give (a) relative time (relative to the selected TIME_EPOCH and
324 in the selected TIME_UNIT; append t to -JX|x), or (b) absolute
325 time of the form [date]T[clock] (append T to -JX|x). At least
326 one of date and clock must be present; the T is always required.
327 The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
328 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
329 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
330 iters and their type and positions must be exactly as indicated
331 (however, input, output and plot formats are customizable; see
332 gmtdefaults). Special case for the UTM projection: If -C is
333 used and -R is not given then the region is set to coincide with
334 the given UTM zone so as to preserve the full ellipsoidal solu‐
335 tion (See RESTRICTIONS for more information).
336
338 No space between the option flag and the associated arguments.
339 infile(s)
341 input file(s) with 2 or more columns. If no file(s) is given,
342 mapproject will read the standard input.
343 -A[f|b]
345 -A calculates the (forward) azimuth from fixed point lon/lat to
346 each data point. Use -Ab to get back-azimuth from data points
347 to fixed point. Upper case F or B will convert from geodetic to
348 geocentric latitudes and estimate azimuth of geodesics (assuming
349 the current ellipsoid is not a sphere). If no fixed point is
350 given then we compute the azimuth (or back-azimuth) from the
351 previous point.
352 -C Set center of projected coordinates to be at map projection cen‐
354 ter [Default is lower left corner]. Optionally, add offsets in
355 the projected units to be added (or subtracted when -I is set)
356 to (from) the projected coordinates, such as false eastings and
357 northings for particular projection zones [0/0]. The unit used
358 for the offsets is the plot distance unit in effect (see MEA‐
359 SURE_UNIT) unless -F is used, in which case the offsets are
360 always in meters.
361 -D Temporarily override MEASURE_UNIT and use c (cm), i (inch), m
363 (meter), or p (points) instead. Cannot be used with -F.
364 -E Convert from geodetic (lon, lat, height) to Earth Centered Earth
366 Fixed (ECEF) (x,y,z) coordinates (add -I for the inverse conver‐
367 sion). Append datum ID (see -Qd) or give ellipsoid:dx,dy,dz
368 where ellipsoid may be an ellipsoid ID (see -Qe) or given as
369 a[,inv_f], where a is the semi-major axis and inv_f is the
370 inverse flattening (0 if omitted). If datum is - or not given
371 we assume WGS-84.
372 -F Force 1:1 scaling, i.e., output (or input, see -I) data are in
374 actual projected meters. To specify other units, append k (km),
375 m (mile), n (nautical mile), i (inch), c (cm), or p (points).
376 Without -F, the output (or input, see -I) are in the units spec‐
377 ified by MEASURE_UNIT (but see -D).
378 -G Calculate distances along track OR to the optional point set
380 with -Gx0/y0. Append IT(unit), the distance unit; choose among
381 e (m), k (km), m (mile), n (nautical mile), d (spherical
382 degree), c (Cartesian distance using input coordinates) or C
383 (Cartesian distance using projected coordinates). The last unit
384 requires -R and -J to be set. Upper case E, K, M, N, or D will
385 use exact methods for geodesic distances (Rudoe's method for
386 distances in length units and employing geocentric latitudes in
387 degree calculations, assuming the current ellipsoid is not
388 spherical). With no fixed point we calculate cumulate distances
389 along track. To obtain incremental distance between successive
390 points, use -G-. To specify the 2nd point via two extra columns
391 in the input file, choose -G+.
392 -H Input file(s) has header record(s). If used, the default number
394 of header records is N_HEADER_RECS. Use -Hi if only input data
395 should have header records [Default will write out header
396 records if the input data have them]. Blank lines and lines
397 starting with # are always skipped.
398 -I Do the Inverse transformation, i.e. get (longitude,latitude)
400 from (x,y) data.
401 -L Determine the shortest distance from the input data points to
403 the line(s) given in the ASCII multi-segment file line.xy. The
404 distance and the coordinates of the nearest point will be
405 appended to the output as three new columns. Append the dis‐
406 tance unit; choose among e (m), k (km), m (mile), n (nautical
407 mile), d (spherical degree), c (Cartesian distance using input
408 coordinates) or C (Cartesian distance using projected coordi‐
409 nates). The last unit requires -R and -J to be set. A spheri‐
410 cal approximation is used for geographic data. Finally, append
411 + to report the line segment id and the fractional point number
412 instead of lon/lat of the nearest point.
413 -Q List all projection parameters. To only list datums, use -Qd.
415 To only list ellipsoids, use -Qe.
416 -S Suppress points that fall outside the region.
418 -T Coordinate conversions between datums from and to using the
420 standard Molodensky transformation. Use -Th if 3rd input column
421 has height above ellipsoid [Default assumes height = 0, i.e., on
422 the ellipsoid]. Specify datums using the datum ID (see -Qd) or
423 give ellipsoid:dx,dy,dz where ellipsoid may be an ellipsoid ID
424 (see -Qe) or given as a[,inv_f], where a is the semi-major axis
425 and inv_f is the inverse flattening (0 if omitted). If datum is
426 - or not given we assume WGS-84. -T may be used in conjunction
427 with -R -J to change the datum before coordinate projection (add
428 -I to apply the datum conversion after the inverse projection).
429 Make sure that the ELLIPSOID setting is correct for your case.
430 -V Selects verbose mode, which will send progress reports to stderr
432 [Default runs "silently"].
433 -: Toggles between (longitude,latitude) and (latitude,longitude)
435 input and/or output. [Default is (longitude,latitude)]. Append
436 i to select input only or o to select output only. [Default
437 affects both].
438 -bi Selects binary input. Append s for single precision [Default is
440 d (double)]. Uppercase S or D will force byte-swapping.
441 Optionally, append ncol, the number of columns in your binary
442 input file if it exceeds the columns needed by the program. Or
443 append c if the input file is netCDF. Optionally, append
444 var1/var2/... to specify the variables to be read. [Default is
445 2 input columns].
446 -bo Selects binary output. Append s for single precision [Default
448 is d (double)]. Uppercase S or D will force byte-swapping.
449 Optionally, append ncol, the number of desired columns in your
450 binary output file. [Default is same as input].
451 -f Special formatting of input and/or output columns (time or geo‐
453 graphical data). Specify i or o to make this apply only to
454 input or output [Default applies to both]. Give one or more
455 columns (or column ranges) separated by commas. Append T (abso‐
456 lute calendar time), t (relative time in chosen TIME_UNIT since
457 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
458 to each column or column range item. Shorthand -f[i|o]g means
459 -f[i|o]0x,1y (geographic coordinates).
460 -g Examine the spacing between consecutive data points in order to
462 impose breaks in the line. Append x|X or y|Y to define a gap
463 when there is a large enough change in the x or y coordinates,
464 respectively, or d|D for distance gaps; use upper case to calcu‐
465 late gaps from projected coordinates. For gap-testing on other
466 columns use [col]z; if col is not prepended the it defaults to 2
467 (i.e., 3rd column). Append [+|-]gap and optionally a unit u.
468 Regarding optional signs: -ve means previous minus current col‐
469 umn value must exceed |gap to be a gap, +ve means current minus
470 previous column value must exceed gap, and no sign means the
471 absolute value of the difference must exceed gap. For geo‐
472 graphic data (x|y|d), the unit u may be meter [Default], kilome‐
473 ter, miles, or nautical miles. For projected data (X|Y|D),
474 choose from inch, centimeter, meter, or points [Default unit set
475 by MEASURE_UNIT]. Note: For x|y|z with time data the unit is
476 instead controlled by TIME_UNIT. Repeat the option to specify
477 multiple criteria, of which any can be met to produce a line
478 break. Issue an additional -ga to indicate that all criteria
479 must be met instead.
480 -m Multiple segment file(s). Segments are separated by a special
482 record. For ASCII files the first character must be flag
483 [Default is '>']. For binary files all fields must be NaN and
484 -b must set the number of output columns explicitly. By default
485 the -m setting applies to both input and output. Use -mi and
486 -mo to give separate settings to input and output.
487
489 The ASCII output formats of numerical data are controlled by parameters
490 in your .gmtdefaults4 file. Longitude and latitude are formatted
491 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
492 according to D_FORMAT. Be aware that the format in effect can lead to
493 loss of precision in the output, which can lead to various problems
494 downstream. If you find the output is not written with enough preci‐
495 sion, consider switching to binary output (-bo if available) or specify
496 more decimals using the D_FORMAT setting.
497
499 To transform a file with (longitude,latitude) into (x,y) positions in
500 cm on a Mercator grid for a given scale of 0.5 cm per degree, run
501 mapproject lonlatfile -R20/50/12/25 -Jm0.5c > xyfile
503 To transform several 2-column, binary, double precision files with
505 (latitude,longitude) into (x,y) positions in inch on a Transverse Mer‐
506 cator grid (central longitude 75W) for scale = 1:500000 and suppress
507 those points that would fall outside the map area, run
508 mapproject tracks.* -R-80/-70/20/40 -Jt-75/1:500000 -: -S -Di -bo -bi2
510 > tmfile.b
511 To convert the geodetic coordinates (lon, lat, height) in the file
513 old.dat from the NAD27 CONUS datum (Datum ID 131 which uses the
514 Clarke-1866 ellipsoid) to WGS 84, run
515 mapproject old.dat -Th131 > new.dat
517 To compute the closest distance (in km) between each point in the input
519 file quakes.dat and the line segments given in the multi-segment ASCII
520 file coastline.xy, run
521 mapproject quakes.dat -Lcoastline.xy/k > quake_dist.dat
523
525 The rectangular input region set with -R will in general be mapped into
526 a non-rectangular grid. Unless -C is set, the leftmost point on this
527 grid has xvalue = 0.0, and the lowermost point will have yvalue = 0.0.
528 Thus, before you digitize a map, run the extreme map coordinates
529 through mapproject using the appropriate scale and see what (x,y) val‐
530 ues they are mapped onto. Use these values when setting up for digi‐
531 tizing in order to have the inverse transformation work correctly, or
532 alternatively, use awk to scale and shift the (x,y) values before
533 transforming.
534 For some projection, a spherical solution may be used despite the user
535 having selected an ellipsoid. This occurs when the users -R setting
536 implies a region that exceeds the domain in which the ellipsoidal
537 series expansions are valid. These are the conditions: (1) Lambert
538 Conformal Conic (-JL)and Albers Equal-Area (-JB) will use the spherical
539 solution when the map scale exceeds 1.0E7. (2) Transverse Mercator
540 (-JT) and UTM (-JU) will will use the spherical solution when either
541 the west or east boundary given in -R is more than 10 degrees from the
542 central meridian, and (3) same for Cassini (-JC) but with a limit of
543 only 4 degrees.
544
546 GMT will use ellipsoidal formulae if they are implemented and the user
547 have selected an ellipsoid as the reference shape (see ELLIPSOID in
548 gmtdefaults). The user needs to be aware of a few potential pitfalls:
549 (1) For some projections, such as Transverse Mercator, Albers, and
550 Lamberts conformal conic we use the ellipsoidal expressions when the
551 areas mapped are small, and switch to the spherical expressions (and
552 substituting the appropriate auxiliary latitudes) for larger maps. The
553 ellipsoidal formulae are used as follows: (a) Transverse Mercator: When
554 all points are within 10 degrees of central meridian, (b) Conic projec‐
555 tions when longitudinal range is less than 90 degrees, (c) Cassini pro‐
556 jection when all points are within 4 degrees of central meridian. (2)
557 When you are trying to match some historical data (e.g., coordinates
558 obtained with a certain projection and a certain reference ellipsoid)
559 you may find that GMT gives results that are slightly different. One
560 likely source of this mismatch is that older calculations often used
561 less significant digits. For instance, Snyder's examples often use the
562 Clarke 1866 ellipsoid (defined by him as having a flattening f =
563 1/294.98). From f we get the eccentricity squared to be 0.00676862818
564 (this is what GMT uses), while Snyder rounds off and uses 0.00676866.
565 This difference can give discrepancies of several tens of cm. If you
566 need to reproduce coordinates projected with this slightly different
567 eccentricity, you should specify your own ellipsoid with the same
568 parameters as Clarke 1866, but with f = 1/294.97861076. Also, be aware
569 that older data may be referenced to different datums, and unless you
570 know which datum was used and convert all data to a common datum you
571 may experience mismatches of tens to hundreds of meters. (3) Finally,
572 be aware that MAP_SCALE_FACTOR have certain default values for some
573 projections so you may have to override the setting in order to match
574 results produced with other settings.
575
577 gmtdefaults(1), GMT(1), project(1)
578
580 Bomford, G., 1952, Geodesy, Oxford U. Press.
581 Snyder, J. P., 1987, Map Projections - A Working Manual, U.S. Geologi‐
582 cal Survey Prof. Paper 1395.
583 Vanicek, P. and Krakiwsky, E, 1982, Geodesy - The Concepts, North-Hol‐
584 land Publ., ISBN: 0 444 86149 1.
585GMT 4.5.6 10 Mar 2011 MAPPROJECT(1)