CanviPrj: Change of Geographic Reference System (projection, datum, etc)

Access this help text as a web page: CanviPrj

Presentation and options	Dialog box of the application
Syntax

Presentation and options

Cartographic information attempts to express measurable characteristics or parameters in our three-dimensional world on paper or digital maps that are typically two-dimensional. To achieve this, projection systems are used. In cartography a projection system is a coordinate transformation from the Earth three-dimensional system to a bidimensional surface. The most important properties a projection system should preserve throughout the map are: scale, shape (conformal) and area (equivalent or equal-area). However, it is impossible to define a projection system having the three properties at the same time and, therefore, the optimal projection depends on the region that is going to be mapped and on the application that the map will have. For example, it is recommended to use UTM projection for local precise applications, and Goode Homolosine projection for global scale maps. Mercator or Lambert Conformal Conic aresuitable for navigation applications (in this use directions are important)while Lambert Azimuthal Equal is more proper for thematic applications.

A reference system is defined by a projection, a set of parameters specified for the projection, a datum and an ellipsoid. For example, in the UTM projection, the zone and the hemisphere must be specified. This way, for Catalonia it will be used the reference system UTM-31N and ETRS89 datum, while for Portugal the UTM-29N will be used and for Lesotho the UTM-35S will beused.

It is very useful to have all the maps that are used in the samereference system, so that it is easy to combine information in the Geographic Information System (GIS). In fact, most of digital GIS allow to operate with different information layers only if they are in the same reference system. In digital systems it is possible to develop a routine to convert a map from one reference system to another, such it is CanviPrj of MiraMon.

This application allows to transform from one projection system to another and also, to change the reference system by setting the projection parameters, the datum, the ellipsoid, etc. It works on raster files (IMG), non structured vector files (VEC) and structured vector files of points (PNT), arcs (ARC) and polygons (POL). To transform node (NOD) files, it is required to transform their associated arc files.

CanviPrj IMG

CanviPrj Structured Vectors

CanviPrj Non Structured Vectors

If the origin map has no reference system it isbecause it has not yet been georeferenced and it is necessary to georeference it with the programCorrGeom.

The description of a reference system involves a great number of parameters. These parameters are in some geodesic tables or could appear directly specified on the REL file. The definition of a reference system is a delicate process but it has to be performed only once by each system; in addition, MiraMon is distributed with a set of definitions of reference systems for common use. The document MiraMon horizontal reference systems. Geodesic tables and files can be consulted for further information.

Changes of reference system in the same datum

The program adapts the complexity of the change of the reference system to the necessary transformation. This way, the datum change functionality appears automatically when the initial and final referential systems are described over a different datum. Typically, we will transform from local datums like the European Datum 1950 (ED50) to global datums like World Geodetic System 1984 (WGS84) or vice-versa.

An exceptional case is when a change of datum is done between datums of the same group. For example, when changing from cartography performed by Institut Catogràfic de Catalunya (ICC) over ED50 but documented with the datum change parameters of European 1950 ICC to cartography performed by Instituto Geográfico Nacional (IGN), documented with the datum change parameters of European 1950 PS. In this case, it could be stated if a change of datum is desired when transforming the 2 systems or if instead, both datums are considered totally equivalent. The parameter "/DATUM_ESTRICTE" allows forcing this change of datum. For cartography over Catalonia, the planimetric difference between choosing the parameter or not is from around 10 meters. Initially, the change between the cartography of ICC and IGN should not be forced because both cartographies work legally over the same datum; but it should be done between French and Spanish cartography in ED50 since, beforehand, they should not be supposed to be exactly the same datums because they depend on different geodesic networks. For French cartography, European 1950 ADFG can be used.

More information can be consulted at the following references:

Pesquer L, Pons X, Masó J (2005) Necesidad de cálculos geodésicos para las herramientas SIG de análisis de distancias y superficies. 6th Geomatic Week proceedings (ISSN 1699-3489). Barcelona.

Dialog box of the application

CanviPrj dialog box

Syntax

Syntax:

CanviPrj IMGFile v_f OutputFileIMG RefSystem int xMin xMax yMin yMax res [/DATUM_ESTRICTE]
CanviPrj EstructVecFile StructOuputFile RefSystem [/DATUM_ESTRICTE]
CanviPrj VecFile VecOutputFile RefSystem f_i.precisio [/DATUM_ESTRICTE]

Parameters:

IMGFile (Input raster file - Input parameter): Input raster file (with extension).
v_f (Flag value - Input parameter): Flag value when the input file (InpFile) has no data to fill output values. 'x' has to be typed for the default value (0).
OutputFileIMG (Raster output file - Output parameter): Raster output file.
RefSystem (Reference system - Input parameter): Reference system of the output image. The available reference systems are in the field ID_GEODES of the m_geodes.dbf or u_geodes.dbf table.
int (Interpolation type - Input parameter): Criterion to decide the output pixel value (Interpolation):
- x: Ask interactively.
- 0: Nearest neighbor (suitable for thematic maps since it preserves the original radiometry).
- 1: Bilineal interpolation (suitable for images for photointerpretation, digital elevation models, etc).
- 2: Bicubic interpolation (as 1 with smoothed results, but slower).
xMin (X minimum - Input parameter): Coordinate X minimum of the output raster bounding. 'x' should be typed for calculate it.
xMax (X maximum - Input parameter): Coordinate X maximum of the output raster bounding. 'x' should be typed for calculate it.
yMin (Y minimum - Input parameter): Coordinate Y minimum of the output raster bounding. 'x' should be typed for calculate it.
yMax (Y maximum - Input parameter): Coordinate Y maximum of the output raster bounding. 'x' should be typed for calculate it.
res (Output image resolution - Input parameter): Output image resolution, in map units. If 'x' is typed, the input resolution is used.

EstructVecFile (Structured input file - Input parameter): Input structured vector file (with extension), of point type (f_i.PNT), arc (f_i.ARC) or polygon (f_i.POL).
StructOuputFile (Structured ouput file - Output parameter): Structured vector ouput file.

VecFile (Input file - Input parameter): Input non structured vector file (with extension) (f_i.VEC).
VecOutputFile (VEC output file - Output parameter): Output non-structured file.
f_i.precisio (Precision - Input parameter): Number of decimal figures of the output vector coordinates (by default 3).

Modifiers:

DATUM_ESTRICTE

Examples:

CanviPrj MUNI_LL.VEC  MUNI_UTM UTM-31N  1
CanviPrj CATA_UTM.IMG CATA2_LL lat/long-WGS84 1 450000 480500 4724500 4750000 30 0
CanviPrj CATA_UTM.IMG CATA3    UTM-31N-UB/ICC   x    x      x      x        x    x x
CanviPrj MA89_UTM.ARC MA89_LCC Lambert_Conformal_Conic-ZoneIII-ED50