| 5 | == Vertical CS in GeoTIFF == |
| 6 | |
| 7 | The planned approach is to store vertical coordinate system in GeoTIFF tags. The GeoTIFF specification has always had vertical coordinate system support, tied to the EPSG codes for vertical coordinate systems and datums. However, traditionally, the vertical coordinate system support in GeoTIFF has also not been widely used. So it is hoped that formalization of GeoTIFF tags for vertical datums in LAS format would also apply for GeoTIFF in general and any other geotiff oriented formats (GeoJP2 for instance). It is anticipated that the GeoTIFF representation of a LAS file with a vertical datum set might look something like this: |
| 8 | |
| 9 | {{{ |
| 10 | Geotiff_Information: |
| 11 | Version: 1 |
| 12 | Key_Revision: 1.0 |
| 13 | Tagged_Information: |
| 14 | End_Of_Tags. |
| 15 | Keyed_Information: |
| 16 | GTModelTypeGeoKey (Short,1): ModelTypeProjected |
| 17 | GTRasterTypeGeoKey (Short,1): RasterPixelIsPoint |
| 18 | ProjectedCSTypeGeoKey (Short,1): PCS_WGS84_UTM_zone_15N |
| 19 | ProjLinearUnitsGeoKey (Short,1): Linear_Meter |
| 20 | VerticalCSTypeGeoKey (Short,1): Unknown-5701 |
| 21 | VerticalCitationGeoKey (Ascii,7): "Newlyn" |
| 22 | VerticalDatumGeoKey (Short,1): Unknown-5101 |
| 23 | VerticalUnitsGeoKey (Short,1): Linear_Meter |
| 24 | End_Of_Keys. |
| 25 | End_Of_Geotiff. |
| 26 | }}} |
| 27 | |
| 28 | Note that VerticalCSTypeGeoKey 5701 is "ODN height", a vertical coordinate system from the EPSG coordinate reference system table. The vertical |
| 29 | coordinate system 5701 is based on vertical datum 5101 (Ordnance Datum Newlyn) from the EPSG datum table. The 5701 vertical coordinate system also specifies use of meters as the vertical linear measure. |
| 30 | |
| 31 | So there are four GeoTIFF tags related to vertical coordinate system information: |
| 32 | * VerticalCSTypeGeoKey: an EPSG coordinate system code of type vertical |
| 33 | (normally 5600-5799). |
| 34 | * !VerticalCitationGeoKey: a description of the vertical coordinate system, |
| 35 | often the EPSG user name for the VerticalCS. |
| 36 | * !VerticalDatumGeoKey: An EPSG vertical datum code (normally 5000 to 5299). |
| 37 | * !VerticalUnitsGeoKey: An EPSG units code (normally 9000 to 9199, 9001=meter) |
| 38 | |
| 39 | Interesting things to note: |
| 40 | * There does not appear to be a !VerticalUnitSizeGeoKey allowing user defined vertical units similarly to the !ProjLinearSizeGeoKey, so it is not appropriate to use !KvUserDefined for !VerticalUnitsGeoKey. |
| 41 | * The libgeotiff epsg_vertcs.inc file seems to imply that the vertical datum codes in the range 5000 to 5299 should be considered vertical coordinate systems which they are not in the EPSG data model. It is not clear whether this is something that changed in the EPSG data model after the initiation of the GeoTIFF specification, or if it was an error by the GeoTIFF/libgeotiff authors. This mismatch is why Unknown-5101 above is not reported as Newlyn directly. Spec issue reported at http://trac.osgeo.org/geotiff/ticket/24 |
| 42 | * The EPSG and OGC models include an EPSG code for a composite coordinate system. The composite coordinate system is a collection of a horizontal coordinate system (like PCS_WGS84_UTM_zone_15N) and a vertical coordinate system (like 5701 - ODN height). An example is [http://spatialreference.org/ref/epsg/7405/html EPSG:7405] - OSGB36 / British National Grid + ODN. There is no way to represent this in a GeoTIFF tag. |