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|Flon0/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] ] [ -M[i|o][flag] ] [ -Q[d|e] [ -S ] [ -T[h]from[/to]
13 ] [ -V ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [
14 -f[i|o]colinfo ]
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. 63 com‐
50 monly used ellipsoids and a spheroid are currently supported,
51 and users may also specify their own 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 -Jylon0/lat0/scale or -JYlon0/lat0/width (Cylindrical Equal-Area
137 [E]).
138 Give the central meridian lon0, standard parallel lat0,
139 and scale (1:xxxx or UNIT/degree). The standard parallel
140 is typically one of these (but can be any value):
141 50 - Balthasart
142 45 - Gall-Peters
143 37.0666 - Caster
144 37.4 - Trystan Edwards
145 37.5 - Hobo-Dyer
146 30 - Behrman
147 0 - Lambert
148 CONIC PROJECTIONS:
150 -Jblon0/lat0/lat1/lat2/scale or -JBlon0/lat0/lat1/lat2/width
152 (Albers [E]).
153 Give projection center lon0/lat0, two standard parallels
154 lat1/lat2, and scale (1:xxxx or UNIT/degree).
155 -Jdlon0/lat0/lat1/lat2/scale or -JDlon0/lat0/lat1/lat2/width
157 (Conic Equidistant)
158 Give projection center lon0/lat0, two standard parallels
159 lat1/lat2, and scale (1:xxxx or UNIT/degree).
160 -Jllon0/lat0/lat1/lat2/scale or -JLlon0/lat0/lat1/lat2/width
162 (Lambert [C])
163 Give origin lon0/lat0, two standard parallels lat1/lat2,
164 and scale along these (1:xxxx or UNIT/degree).
165 AZIMUTHAL PROJECTIONS:
167 Except for polar aspects, -Rw/e/s/n will be reset to -Rg. Use
169 -R<...>r for smaller regions.
170 -Jalon0/lat0[/horizon]/scale or -JAlon0/lat0[/horizon]/width
172 (Lambert [E]).
173 lon0/lat0 specifies the projection center. horizon spec‐
174 ifies the max distance from projection center (in
175 degrees, <= 180, default 90). Give scale as 1:xxxx or
176 radius/lat, where radius is distance in UNIT from origin
177 to the oblique latitude lat.
178 -Jelon0/lat0[/horizon]/scale or -JElon0/lat0[/horizon]/width
180 (Azimuthal Equidistant).
181 lon0/lat0 specifies the projection center. horizon spec‐
182 ifies the max distance from projection center (in
183 degrees, <= 180, default 180). Give scale as 1:xxxx or
184 radius/lat, where radius is distance in UNIT from origin
185 to the oblique latitude lat.
186 -Jflon0/lat0[/horizon]/scale or -JFlon0/lat0[/horizon]/width
188 (Gnomonic).
189 lon0/lat0 specifies the projection center. horizon spec‐
190 ifies the max distance from projection center (in
191 degrees, < 90, default 60). Give scale as 1:xxxx or
192 radius/lat, where radius is distance in UNIT from origin
193 to the oblique latitude lat.
194 -Jglon0/lat0[/horizon]/scale or -JGlon0/lat0[/horizon]/width
196 (Orthographic).
197 lon0/lat0 specifies the projection center. horizon spec‐
198 ifies the max distance from projection center (in
199 degrees, <= 90, default 90). Give scale as 1:xxxx or
200 radius/lat, where radius is distance in UNIT from origin
201 to the oblique latitude lat.
202 -Jglon0/lat0/altitude/azimuth/tilt/twist/Width/Height/scale or
204 -JGlon0/lat0/altitude/azimuth/tilt/twist/Width/Height/width
205 (General Perspective).
206 lon0/lat0 specifies the projection center. altitude is
207 the height (in km) of the viewpoint above local sea
208 level. If altitude is less than 10, then it is the dis‐
209 tance from the center of the earth to the viewpoint in
210 earth radii. If altitude has a suffix r then it is the
211 radius from the center of the earth in kilometers.
212 azimuth is measured to the east of north of view. tilt
213 is the upward tilt of the plane of projection. If tilt is
214 negative, then the viewpoint is centered on the horizon.
215 Further, specify the clockwise twist, Width, and Height
216 of the viewpoint in degrees. Give scale as 1:xxxx or
217 radius/lat, where radius is distance in UNIT from origin
218 to the oblique latitude lat.
219 -Jslon0/lat0[/horizon]/scale or -JSlon0/lat0[/horizon]/width
221 (General Stereographic [C]).
222 lon0/lat0 specifies the projection center. horizon spec‐
223 ifies the max distance from projection center (in
224 degrees, < 180, default 90). Give scale as 1:xxxx (true
225 at pole) or lat0/1:xxxx (true at standard parallel lat0)
226 or radius/lat (radius in UNIT from origin to the oblique
227 latitude lat).
228 MISCELLANEOUS PROJECTIONS:
230 -Jh[lon0/]scale or -JH[lon0/]width (Hammer [E]).
232 Give the central meridian lon0 (optional) and scale along
233 equator (1:xxxx or UNIT/degree).
234 -Ji[lon0/]scale or -JI[lon0/]width (Sinusoidal [E]).
236 Give the central meridian lon0 (optional) and scale along
237 equator (1:xxxx or UNIT/degree).
238 -Jkf[lon0/]scale or -JKf[lon0/]width (Eckert IV) [E]).
240 Give the central meridian lon0 (optional) and scale along
241 equator (1:xxxx or UNIT/degree).
242 -Jk[s][lon0/]scale or -JK[s][lon0/]width (Eckert VI) [E]).
244 Give the central meridian lon0 (optional) and scale along
245 equator (1:xxxx or UNIT/degree).
246 -Jn[lon0/]scale or -JN[lon0/]width (Robinson).
248 Give the central meridian lon0 (optional) and scale along
249 equator (1:xxxx or UNIT/degree).
250 -Jr[lon0/]scale -JR[lon0/]width (Winkel Tripel).
252 Give the central meridian lon0 (optional) and scale along
253 equator (1:xxxx or UNIT/degree).
254 -Jv[lon0/]scale or -JV[lon0/]width (Van der Grinten).
256 Give the central meridian lon0 (optional) and scale along
257 equator (1:xxxx or UNIT/degree).
258 -Jw[lon0/]scale or -JW[lon0/]width (Mollweide [E]).
260 Give the central meridian lon0 (optional) and scale along
261 equator (1:xxxx or UNIT/degree).
262 NON-GEOGRAPHICAL PROJECTIONS:
264 -Jp[a]scale[/origin][r|z] or -JP[a]width[/origin][r|z] (Polar
266 coordinates (theta,r))
267 Optionally insert a after -Jp [ or -JP] for azimuths CW
268 from North instead of directions CCW from East [Default].
269 Optionally append /origin in degrees to indicate an angu‐
270 lar offset [0]). Finally, append r if r is elevations in
271 degrees (requires s >= 0 and n <= 90) or z if you want to
272 annotate depth rather than radius [Default]. Give scale
273 in UNIT/r-unit.
274 -Jxx-scale[/y-scale] or -JXwidth[/height] (Linear, log, and
276 power scaling)
277 Give x-scale (1:xxxx or UNIT/x-unit) and/or y-scale
278 (1:xxxx or UNIT/y-unit); or specify width and/or height
279 in UNIT. y-scale=x-scale if not specified separately and
280 using 1:xxxx implies that x-unit and y-unit are in
281 meters. Use negative scale(s) to reverse the direction
282 of an axis (e.g., to have y be positive down). Option‐
283 ally, append to x-scale, y-scale, width or height one of
284 the following:
285 d Data are geographical coordinates (in degrees).
287 l Take log10 of values before scaling.
289 ppower Raise values to power before scaling.
291 t Input coordinates are time relative to TIME_EPOCH.
293 T Input coordinates are absolute time.
295 Default axis lengths (see gmtdefaults) can be invoked
297 using -JXh (for landscape); -JXv (for portrait) will swap
298 the x- and y-axis lengths. The default unit for this
299 installation is either cm or inch, as defined in the file
300 share/gmt.conf. However, you may change this by editing
301 your .gmtdefaults4 file(s).
302 -R xmin, xmax, ymin, and ymax specify the Region of interest. For
304 geographic regions, these limits correspond to west, east,
305 south, and north and you may specify them in decimal degrees or
306 in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left
307 and upper right map coordinates are given instead of w/e/s/n.
308 The two shorthands -Rg and -Rd stand for global domain (0/360
309 and -180/+180 in longitude respectively, with -90/+90 in lati‐
310 tude). For calendar time coordinates you may either give (a)
311 relative time (relative to the selected TIME_EPOCH and in the
312 selected TIME_UNIT; append t to -JX|x), or (b) absolute time of
313 the form [date]T[clock] (append T to -JX|x). At least one of
314 date and clock must be present; the T is always required. The
315 date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian
316 calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock
317 string must be of the form hh:mm:ss[.xxx]. The use of delim‐
318 iters and their type and positions must be exactly as indicated
319 (however, input, output and plot formats are customizable; see
320 gmtdefaults). Special case for the UTM projection: If -C is
321 used and -R is not given then the region is set to coincide with
322 the given UTM zone so as to preserve the full ellipsoidal solu‐
323 tion (See RESTRICTIONS for more information).
324
326 No space between the option flag and the associated arguments.
327 infile(s)
329 input file(s) with 2 or more columns. If no file(s) is given,
330 mapproject will read the standard input.
331 -A[f|b]
333 -A calculates the (forward) azimuth from fixed point lon/lat to
334 each data point. Use -Ab to get back-azimuth from data points
335 to fixed point. Upper case F or B will convert from geodetic to
336 geocentric latitudes and estimate azimuth of geodesics (assuming
337 the current ellipsoid is not a sphere).
338 -C Set center of projected coordinates to be at map projection cen‐
340 ter [Default is lower left corner]. Optionally, add offsets in
341 the projected units to be added (or subtracted when -I is set)
342 to (from) the projected coordinates, such as false eastings and
343 northings for particular projection zones [0/0]. The unit used
344 for the offsets is the plot distance unit in effect (see MEA‐
345 SURE_UNIT) unless -F is used, in which case the offsets are in
346 meters.
347 -D Temporarily override MEASURE_UNIT and use c (cm), i (inch), m
349 (meter), or p (points) instead. Cannot be used with -F.
350 -E Convert from geodetic (lon, lat, height) to Earth Centered Earth
352 Fixed (ECEF) (x,y,z) coordinates (add -I for the inverse conver‐
353 sion). Append datum ID (see -Qd) or give ellipsoid:dx,dy,dz
354 where ellipsoid may be an ellipsoid ID (see -Qe) or given as
355 a,1/f. If datum is - or not given we assume WGS-84.
356 -F Force 1:1 scaling, i.e., output (or input, see -I) data are in
358 actual projected meters. To specify other units, append k (km),
359 m (mile), n (nautical mile), i (inch), c (cm), or p (points).
360 Without -F, the output (or input, see -I) are in the units spec‐
361 ified by MEASURE_UNIT (but see -D).
362 -G Calculate distances along track OR to the optional point set
364 with -Gx0/y0. Append the distance unit; choose among e (m), k
365 (km), m (mile), n (nautical mile), d (spherical degree), c
366 (Cartesian distance using input coordinates) or C (Cartesian
367 distance using projected coordinates). The last unit requires
368 -R and -J to be set. Upper case E, K, M, N, or D will use
369 exact methods for geodesic distances (Rudoe's method for dis‐
370 tances in length units and employing geocentric latitudes in
371 degree calculations, assuming the current ellipsoid is not a
372 sphere). With no fixed point we calculate cumulate distances
373 along track. To obtain incremental distance between successive
374 points, use -G-. To specify the 2nd point via two extra columns
375 in the input file, choose -G+.
376 -H Input file(s) has Header record(s). Number of header records
378 can be changed by editing your .gmtdefaults4 file. If used, GMT
379 default is 1 header record. Use -Hi if only input data should
380 have header records [Default will write out header records if
381 the input data have them]. Blank lines and lines starting with #
382 are always skipped.
383 -I Do the Inverse transformation, i.e. get (longitude,latitude)
385 from (x,y) data.
386 -L Determine the shortest distance from the input data points to
388 the line(s) given in the ASCII multi-segment file line.xy. The
389 distance and the coordinates of the nearest point will be
390 appended to the output as three new columns. Append the dis‐
391 tance unit; choose among e (m), k (km), m (mile), n (nautical
392 mile), d (spherical degree), c (Cartesian distance using input
393 coordinates) or C (Cartesian distance using projected coordi‐
394 nates). The last unit requires -R and -J to be set. A spheri‐
395 cal approximation is used for geographic data.
396 -M Multiple segment file(s). Segments are separated by a special
398 record. For ASCII files the first character must be flag
399 [Default is '>']. For binary files all fields must be NaN and
400 -b must set the number of output columns explicitly. By default
401 the -M setting applies to both input and output. Use -Mi and
402 -Mo to give separate settings.
403 -Q List all projection parameters. To only list datums, use -Qd.
405 To only list ellipsoids, use -Qe.
406 -S Suppress points that fall outside the region.
408 -T Coordinate conversions between datums from and to using the
410 standard Molodensky transformation. Use -Th if 3rd input column
411 has height above ellipsoid [Default assumes height = 0, i.e., on
412 the ellipsoid]. Specify datums using the ID (see -Qd) or give
413 ellipsoid:dx,dy,dz, where ellipsoid may be an ellipsoid ID (see
414 -Qe) or given as a,1/f. If datum is - or not given we use
415 WGS-84. -T may be used in conjunction with -R -J to change the
416 datum before coordinate projection (add -I to apply the datum
417 conversion after the inverse projection). Make sure that the
418 ELLIPSOID setting is correct for your case.
419 -V Selects verbose mode, which will send progress reports to stderr
421 [Default runs "silently"].
422 -: Toggles between (longitude,latitude) and (latitude,longitude)
424 input and/or output. [Default is (longitude,latitude)]. Append
425 i to select input only or o to select output only. [Default
426 affects both].
427 -bi Selects binary input. Append s for single precision [Default is
429 d (double)]. Uppercase S or D will force byte-swapping.
430 Optionally, append ncol, the number of columns in your binary
431 input file if it exceeds the columns needed by the program. Or
432 append c if the input file is netCDF. Optionally, append
433 var1/var2/... to specify the variables to be read. [Default is
434 2 input columns].
435 -bo Selects binary output. Append s for single precision [Default
437 is d (double)]. Uppercase S or D will force byte-swapping.
438 Optionally, append ncol, the number of desired columns in your
439 binary output file. [Default is same as input].
440 -f Special formatting of input and/or output columns (time or geo‐
442 graphical data). Specify i or o to make this apply only to
443 input or output [Default applies to both]. Give one or more
444 columns (or column ranges) separated by commas. Append T (abso‐
445 lute calendar time), t (relative time in chosen TIME_UNIT since
446 TIME_EPOCH), x (longitude), y (latitude), or f (floating point)
447 to each column or column range item. Shorthand -f[i|o]g means
448 -f[i|o]0x,1y (geographic coordinates).
449
451 The ASCII output formats of numerical data are controlled by parameters
452 in your .gmtdefaults4 file. Longitude and latitude are formatted
453 according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
454 according to D_FORMAT. Be aware that the format in effect can lead to
455 loss of precision in the output, which can lead to various problems
456 downstream. If you find the output is not written with enough preci‐
457 sion, consider switching to binary output (-bo if available) or specify
458 more decimals using the D_FORMAT setting.
459
461 To transform a file with (longitude,latitude) into (x,y) positions in
462 cm on a Mercator grid for a given scale of 0.5 cm per degree, run
463 mapproject lonlatfile -R20/50/12/25 -Jm0.5c > xyfile
465 To transform several 2-column, binary, double precision files with
467 (latitude,longitude) into (x,y) positions in inch on a Transverse Mer‐
468 cator grid (central longitude 75W) for scale = 1:500000 and suppress
469 those points that would fall outside the map area, run
470 mapproject tracks.* -R-80/-70/20/40 -Jt-75/1:500000 -: -S -Di -bo -bi2
472 > tmfile.b
473 To convert the geodetic coordinates (lon, lat, height) in the file
475 old.dat from the NAD27 CONUS datum (Datum ID 131 which uses the
476 Clarke-1866 ellipsoid) to WGS 84, run
477 mapproject old.dat -Th131 > new.dat
479 To compute the closest distance (in km) between each point in the input
481 file quakes.dat and the line segments given in the multi-segment ASCII
482 file coastline.xy, run
483 mapproject quakes.dat -Lcoastline.xy/k > quake_dist.dat
485
487 The rectangular input region set with -R will in general be mapped into
488 a non-rectangular grid. Unless -C is set, the leftmost point on this
489 grid has xvalue = 0.0, and the lowermost point will have yvalue = 0.0.
490 Thus, before you digitize a map, run the extreme map coordinates
491 through mapproject using the appropriate scale and see what (x,y) val‐
492 ues they are mapped onto. Use these values when setting up for digi‐
493 tizing in order to have the inverse transformation work correctly, or
494 alternatively, use awk to scale and shift the (x,y) values before
495 transforming.
496 FOr some projection, a spherical solution may be used despite the user
497 having selected an ellipsoid. This occurs when the users -R setting
498 implies a region that exceeds the domain in which the ellipsoidal
499 series expansions are valid. These are the conditions: (1) Lambert
500 Conformal Conic (-JL)and Albers Equal-Area (-JB) will use the spherical
501 solution when the map scale exceeds 1.0E7. (2) Transverse Mercator
502 (-JT) and UTM (-JU) will will use the spherical solution when either
503 the west or east boundary given in -R is more than 10 degrees from the
504 central meridian, and (3) same for Cassini (-JC) but with a limit of
505 only 4 degrees.
506
508 GMT will use ellipsoidal formulae if they are implemented and the user
509 have selected an ellipsoid as the reference shape (see ELLIPSOID in
510 gmtdefaults). The user needs to be aware of a few potential pitfalls:
511 (1) For some projections, such as Transverse Mercator, Albers, and
512 Lamberts conformal conic we use the ellipsoidal expressions when the
513 areas mapped are small, and switch to the spherical expressions (and
514 substituting the appropriate auxiliary latitudes) for larger maps. The
515 ellipsoidal formulae are used as follows: (a) Transverse Mercator: When
516 all points are within 10 degrees of central meridian, (b) Conic projec‐
517 tions when longitudinal range is less than 90 degrees, (c) Cassini pro‐
518 jection when all points are within 4 degrees of central meridian. (2)
519 When you are trying to match some historical data (e.g., coordinates
520 obtained with a certain projection and a certain reference ellipsoid)
521 you may find that GMT gives results that are slightly different. One
522 likely source of this mismatch is that older calculations often used
523 less significant digits. For instance, Snyder's examples often use the
524 Clarke 1866 ellipsoid (defined by him as having a flattening f =
525 1/294.98). From f we get the eccentricity squared to be 0.00676862818
526 (this is what GMT uses), while Snyder rounds off and uses 0.00676866.
527 This difference can give discrepancies of several tens of cm. If you
528 need to reproduce coordinates projected with this slightly different
529 eccentricity, you should specify your own ellipsoid with the same
530 parameters as Clarke 1866, but with f = 1/294.97861076. Also, be aware
531 that older data may be referenced to different datums, and unless you
532 know which datum was used and convert all data to a common datum you
533 may experience mismatches of tens to hundreds of meters. (3) Finally,
534 be aware that MAP_SCALE_FACTOR have certain default values for some
535 projections so you may have to override the setting in order to match
536 results produced with other settings.
537
539 gmtdefaults(1), GMT(1), project(1)
540
542 Bomford, G., 1952, Geodesy, Oxford U. Press.
543 Snyder, J. P., 1987, Map Projections - A Working Manual, U.S. Geologi‐
544 cal Survey Prof. Paper 1395.
545 Vanicek, P. and Krakiwsky, E, 1982, Geodesy - The Concepts, North-Hol‐
546 land Publ., ISBN: 0 444 86149 1.
547GMT 4.3.1 15 May 2008 MAPPROJECT(1)