1r.proj(1) Grass User's Manual r.proj(1)
2
6 r.proj - Re-projects a raster map from one location to the current
7 location.
8
10 raster, projection
11
13 r.proj
14 r.proj help
15 r.proj [-ln] [input=name] location=string [mapset=string]
16 [dbase=string] [output=name] [method=string] [memory=integer]
17 [resolution=float] [--overwrite] [--verbose] [--quiet]
18 Flags:
20 -l
21 List raster maps in input location and exit
22 -n
24 Do not perform region cropping optimization
25 --overwrite
27 Allow output files to overwrite existing files
28 --verbose
30 Verbose module output
31 --quiet
33 Quiet module output
34 Parameters:
36 input=name
37 Name of input raster map to re-project
38 location=string
40 Location of input raster map
41 mapset=string
43 Mapset of input raster map
44 dbase=string
46 Path to GRASS database of input location
47 output=name
49 Name for output raster map (default: input)
50 method=string
52 Interpolation method to use
53 Options: nearest,bilinear,cubic
54 Default: nearest
55 memory=integer
57 Cache size (MiB)
58 resolution=float
60 Resolution of output map
61
63 r.proj projects a raster map in a specified mapset of a specified loca‐
64 tion from the projection of the input location to a raster map in the
65 current location. The projection information is taken from the current
66 PROJ_INFO files, as set with g.setproj
67 and viewed with g.proj.
68 Introduction
70 Map projections Map projections are a method of representing informa‐
71 tion from a curved surface (usually a spheroid) in two dimensions, typ‐
72 ically to allow indexing through cartesian coordinates. There are a
73 wide variety of projections, with common ones divided into a number of
74 classes, including cylindrical and pseudo-cylindrical, conic and
75 pseudo-conic, and azimuthal methods, each of which may be conformal,
76 equal-area, or neither.
77 The particular projection chosen depends on the purpose of the project,
79 and the size, shape and location of the area of interest. For example,
80 normal cylindrical projections are good for maps which are of greater
81 extent east-west than north-south and in equatorial regions, while
82 conic projections are better in mid-latitudes; transverse cylindrical
83 projections are used for maps which are of greater extent north-south
84 than east-west; azimuthal projections are used for polar regions.
85 Oblique versions of any of these may also be used. Conformal projec‐
86 tions preserve angular relationships, and better preserve arc-length,
87 while equal-area projections are more appropriate for statistical stud‐
88 ies and work in which the amount of material is important.
89 Projections are defined by precise mathematical relations, so the
91 method of projecting coordinates from a geographic reference frame
92 (latitude-longitude) into a projected cartesian reference frame (eg
93 metres) is governed by these equations. Inverse projections can also
94 be achieved. The public-domain Unix software package PROJ [1] has been
95 designed to perform these transformations, and the user's manual con‐
96 tains a detailed description of over 100 useful projections. This also
97 includes a programmers library of the projection methods to support
98 other software development.
99 Thus, converting a vector map - in which objects are located with arbi‐
101 trary spatial precision - from one projection into another is usually
102 accomplished by a simple two-step process: first the location of all
103 the points in the map are converted from the source through an inverse
104 projection into latitude-longitude, and then through a forward projec‐
105 tion into the target. (Of course the procedure will be one-step if
106 either the source or target is in geographic coordinates.)
107 Converting a raster map, or image, between different projections, how‐
109 ever, involves additional considerations. A raster may be considered
110 to represent a sampling of a process at a regular, ordered set of loca‐
111 tions. The set of locations that lie at the intersections of a carte‐
112 sian grid in one projection will not, in general, coincide with the
113 sample points in another projection. Thus, the conversion of raster
114 maps involves an interpolation step in which the values of points at
115 intermediate locations relative to the source grid are estimated. Pro‐
116 jecting vector maps within the GRASS GIS GIS data capture, import and
117 transfer often requires a projection step, since the source or client
118 will frequently be in a different projection to the working projection.
119 In some cases it is convenient to do the conversion outside the pack‐
121 age, prior to import or after export, using software such as PROJ.4's
122 cs2cs [1]. This is an easy method for converting an ASCII file contain‐
123 ing a list of coordinate points, since there is no topology to be pre‐
124 served and cs2cs can be used to process simple lists using a one-line
125 command.
126 The format of files containing vector maps with lines and arcs is gen‐
128 erally more complex, as parts of the data stored in the files will
129 describe topology, and not just coordinates. In GRASS GIS the v.proj
130 module is provided to reproject vector maps, transferring topology and
131 attributes as well as node coordinates. This program uses the projec‐
132 tion definition and parameters which are stored in the PROJ_INFO and
133 PROJ_UNITS files in the PERMANENT mapset directory for every GRASS
134 location.
135 Design of r.proj
137 As discussed briefly above, the fundamental step in re-projecting a
138 raster is resampling the source grid at locations corresponding to the
139 intersections of a grid in the target projection. The basic procedure
140 for accomplishing this, therefore, is as follows:
141 r.proj converts a map to a new geographic projection. It reads a map
143 from a different location, projects it and write it out to the current
144 location.
145 The projected data is resampled with one of three different methods:
146 nearest neighbor, bilinear and cubic convolution.
147 The method=nearest method, which performs a nearest neighbor assign‐
149 ment, is the fastest of the three resampling methods. It is primarily
150 used for categorical data such as a land use classification, since it
151 will not change the values of the data cells. The method=bilinear
152 method determines the new value of the cell based on a weighted dis‐
153 tance average of the 4 surrounding cells in the input map. The
154 method=cubic method determines the new value of the cell based on a
155 weighted distance average of the 16 surrounding cells in the input map.
156 The bilinear and cubic interpolation methods are most appropriate for
158 continuous data and cause some smoothing. Both options should not be
159 used with categorical data, since the cell values will be altered.
160 If nearest neighbor assignment is used, the output map has the same
162 raster format as the input map. If any of the both interpolations is
163 used, the output map is written as floating point.
164 Note that, following normal GRASS conventions, the coverage and resolu‐
166 tion of the resulting grid is set by the current region settings, which
167 may be adjusted using g.region. The target raster will be relatively
168 unbiased for all cases if its grid has a similar resolution to the
169 source, so that the resampling/interpolation step is only a local oper‐
170 ation. If the resolution is changed significantly, then the behaviour
171 of the generalisation or refinement will depend on the model of the
172 process being represented. This will be very different for categorical
173 versus numerical data. Note that three methods for the local interpo‐
174 lation step are provided.
175 r.proj supports general datum transformations, making use of the PROJ.4
177 co-ordinate system translation library.
178
180 To avoid excessive time consumption when reprojecting a map the region
181 and resolution of the target location should be set appropriately
182 beforehand. A simple way to do this is to generate a vector "box" map
183 of the region in the source location using v.in.region. This "box" map
184 is then reprojected into the target location with v.proj. Next the
185 region in the target location is set to the extent of the new vector
186 map with g.region along with the desired raster resolution (g.region -m
187 can be used in Latitude/Longitude locations to measure the geodetic
188 length of a pixel). r.proj is then run for the raster map the user
189 wants to reproject. In this case a little preparation goes a long way.
190 When reprojecting whole-world maps the user should disable map-trimming
192 with the -n flag. Trimming is not useful here because the module has
193 the whole map in memory anyway. Besides that, world "edges" are hard
194 (or impossible) to find in projections other than latitude-longitude so
195 results may be odd with trimming.
196
198 [1] Evenden, G.I. (1990) Cartographic projection procedures for the
199 UNIX environment - a user's manual. USGS Open-File Report 90-284
200 (OF90-284.pdf) See also there: Interim Report and 2nd Interim Report on
201 Release 4, Evenden 1994).
202 Richards, John A. (1993), Remote Sensing Digital Image Analysis,
204 Springer-Verlag, Berlin, 2nd edition.
205 PROJ.4: Projection/datum support library.
207 Further reading
209 ASPRS Grids and Datum
211 Projections Transform List (PROJ.4)
213 MapRef - The Collection of Map Projections and Reference
215 Systems for Europe
216 Information and Service System for European Coordinate
218 Reference Systems - CRS
219
221 g.region, g.proj, g.setproj, i.rectify, r.support, r.stats, v.proj,
222 v.in.region
223 The 'gdalwarp' and 'gdal_translate' utilities are available from the
225 GDAL project.
226
228 Martin Schroeder, University of Heidelberg, Germany
229 Man page text from S.J.D. Cox, AGCRC, CSIRO Exploration & Mining, Ned‐
231 lands, WA
232 Updated by Morten Hulden
234 Datum tranformation support and cleanup by Paul Kelly
236 Last changed: $Date: 2007-07-20 09:41:57 +0200 (Fri, 20 Jul 2007) $
238 Full index
240 © 2003-2008 GRASS Development Team
242GRASS 6.3.0 r.proj(1)