# NWT_GRD/NWT_GRC -- Northwood/Vertical Mapper File Format

Support for reading & writing Northwood GRID raster formats. This format is also known as Vertical Mapper Grid or MapInfo Grid and is commonly used in MapInfo Pro software

Full read/write support of *.grd (grid) files is available, read-only support is available for classified grids (*.grc).

For writing, Float32 is the only supported band type.

## Color Information

The grid formats have color information embedded in the grid file header. This describes how to scale data values to RGB values. When opening in read mode, the driver will report 4 bands - R, G, B and the data band. In reality there is 1 band and the RGB bands are 'virtual', made from scaling data. For this reason, when opening in write mode only 1 band is reported and the RGB bands are unavailable.

## Metadata

GDAL Metadata items are stored in the PAM .aux.xml file

Northwood Grid itself does not natively support arbritrary metadata

## Nodata values

In write mode, it is possible to designate any value as the nodata value. These values are translated to the Vertical Mapper no data value when writing. Therefore, in read mode the nodata value is always reported as -1e37.

### Creation Options

- ZMIN=-2e37: Set the minimum Z value. Data are scaled on disk to a 16 bit integer and the Z value range is used to scale data. If not set, it may cause incorrect data to be written when using 'Create()' or a full recalculation of the source dataset statistics when using 'CreateCopy'

- ZMAX=2e38: Set the maximum Z value. Data are scaled on disk to a 16 bit integer and the Z value range is used to scale data. If not set, it may cause incorrect data to be written when using 'Create()' or a full recalculation of the source dataset statistics when using 'CreateCopy'

- BRIGHTNESS=50: Set the brightness level. Only affects opening the file in MapInfo/Vertical Mapper

- CONTRAST=50: Set the contrast level. Only affects opening the file in MapInfo/Vertical Mapper

- TRANSCOLOR=0: Set a transparent color level. Only affects opening the file in MapInfo/Vertical Mapper

- TRANSLUCENCY=0: Set the translucency level. Only affects opening the file in MapInfo/Vertical Mapper