Background

Raster Manager supports on-the-fly reprojection of any geocoded and georeferenced image. 

Reprojection In Raster Manager Dialog

Raster Manager provides three columns of information which are related to raster reprojection.  They are: 

• Inherit GeoCS from Model  for activating the on-the-fly reprojection,
• Geo Priority for selecting the geo priority method, and
• Coordinate System which describes the coordinate system found from the geocoding information.

To display these columns, right-click the title of any column name and enable their display  in the resulting picklist.

Georeferencing 

Georeferencing is the operation of positioning a raster in the world by using the following six parameters :

• Sx: The scale in the X direction.
• Sxy: The scale in the X direction related to Y.
• Sy: The scale in the Y direction.
• Syx: The scale in the Y direction related to X.
• Tx: The translation in the X direction.
• Ty: The translation in the Y direction.

Sister Files

A sister file is a text file stored in the same folder as the image it georeferences.  It has the same name as the image, but has a different extension. A sister file is usually used with raster file that doesn't support georeferencing.

Raster Manager supports the following sister files :

• HGR: The proprietary sister file of Bentley, written as an ASCII file. It can support geometric transformation like translation, scaling, rotation and affinity.
• World File: The popular georeferencing file introduced by ESRI.
• ERS: The sister file developed originally by ErMapper. Contrary to the other two supported sister file, the ERS sister file also supports geocoding.

Setting The Georeference

Specifying the georeference can be done manually by using the basic transformation tool found in the Raster Manager.  For additional information, refer to:  Raster Manager - Basic Raster Editing.  This is not recommended  for precise georeferencing.

When the exact six transformation parameters are known, it's possible to create a sister file and manually write the transformation parameters to it.

A more precise approach is to register the raster with Bentley Descartes.  Please refer to Bentley Descartes - Raster Registration for more information.

Georeference Preferences

Found in the Raster Manager category of the Preferences dialog, georeference preferences can affect the final georeference attributed to a file.

• Use Sister File, if Present, for Georeferenced Files:  It's possible that a file containing georeference information be accompanied by a sister file. When that happens, Raster Manager checks this option to determine if it should use the georeference information presented in the sister file, or the file's georeference information. 
• Save Location Info in Sister File if Required: This option specifies if the location information should be stored in a sister file. 
• Default Unit Settings: It is possible that a raster file or sister file contains no information about the unit of its georeference. In that case, Raster Manager uses the default unit settings. 
• Use Unit Definition Geokey if Present: The geocoding in a GeoTIFF file is represented by a list of keys. Two important keys are ProjectedCSTypeGeoKey and ProjLinearUnitsGeoKey, which respectively defines the EPSG code for the projected coordinate system and the EPSG code of the projected linear units. A EPSG's defined projected coordinate system is related to a projected linear units. In some GeoTIFFs, it is possible that the projected linear units defined for the projected coordinate system is not correct. In that case, the ProjLinearUnitsGeoKey can be used to specify the correct projected linear unit. Raster Manager uses the Use Unit Definition Geokey if Present option when both the ProjectedCSTypeGeoKey and ProjLinearUnitsGeoKey keys are present to determine which key should be use to determine the projected linear unit.

Extra:  EPSG stands for European Petroleum Survey Group  which has defined numeric identifies for common projections and coordinate metadata.  These codes are industry standards and are used with MicroStation as well as other products.

Geocoding

Geocoding is information describing the coordinate system for which the georeference is applied. It's the geocoding information that specifies the unit of the georeference and the type of coordinate such as spherical or planar.

There are two basic kinds of geocoding information :

• Geographic:  Information that is used to related the position of the the raster to latitude / longitude coordinates. The geographic information mostly describe the earth model. 
• Map Projection: Information that is used to described the projection operation that has been applied to the source raster data before creating the raster file.  

For a list of raster file formats supporting geocoding in their header, refer to the section titled Supported File Formats in Raster Manager - Raster File Format Handling.

Setting the Geocoding

The geocoding can be easily set for any attached rasters in the Raster Manager dialog by right-clicking on the raster and selecting the menu item Coordinate System > Select From Library. The Raster Managers Coordinate System column can be used to determine if a coordinate system is already specified for a given raster. Note that the geocoding information is going to be saved to the raster header or the attachment depending on its geo priority value.

Geo Priority

Geo priority is an important concept in Raster Manager.  It determines which source of georeference / geocoding information should be used when multiple sources are available. Raster Manager supports three information sources :

• Raster Header: The georeference is stored in the header of the raster file. See Supported File Formats in Raster Manager - Raster File Format Handling for the list of raster file formats supporting georeference iinformation in their header.
• Sister File: The georeference is stored in an accompanying file.
• Attachment: The georeference is stored in the raster element saved in the design file.

Reprojection

Reprojection is the process of transforming a raster from one coordinate system to another coordinate system.  This allows data sources in different coordinate systems be used together. 

On-the-fly

Raster Manager is able to reproject any georeferenced and geocoded raster on-the-fly without changing the raster file's coordinate system.

The reprojection can be done by simply unchecking the option Inherit GeoCS from Model. Note that the destination coordinate system is always the coordinate system of the model. If the model has no coordinate system, the raster will not be reprojected. Most of the time, the reprojection of a raster will move the raster to another location. 

Performance Tip : Creating a reprojected version of a raster on disk is more efficient than reprojecting the image on-the-fly.  Note that Bentley Descartes is required for this.

Domain Of Validity

Some geographic coordinate systems, meaning those coordinate system with no Map Projection information, and all reprojected coordinate systems have a validity zone.  This is a  zone on the earth for which the coordinate system is valid. For example, some projected coordinate systems can only be used for areas in North America, while others can only be used for European regions.

When reprojecting a raster which is covering a region greater than the validity zone of the destination coordinate system, Raster Manager will automatically clip the reprojected raster so that only the area of the raster in the destination coordinate system's validity zone be displayed. The pictures below show the clipping that is applied to a world image when reprojected to a coordinate system whose validity zone is not the whole world.

Unreprojected World Image

Clipped, Reprojected World Image

See also

 Assigning A Geographic Coordinate System 

 Reprojecting to a new Geographic Coordinate System   

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