Change of height to a different vertical reference surface.
The Baltic 1977 vertical reference surface is 28m above the Caspian vertical reference surface.
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Definition: OGC Well Known Text 2 (2019)
COORDINATEOPERATION["Baltic 1977 depth to Caspian depth (1)", VERSION["Caspian Sea"], SOURCECRS[ VERTCRS["Baltic 1977 depth", VDATUM["Baltic 1977"], CS[vertical,1], AXIS["depth (D)",down, LENGTHUNIT["metre",1]], ID["EPSG",5612]]], TARGETCRS[ VERTCRS["Caspian depth", VDATUM["Caspian Sea"], CS[vertical,1], AXIS["depth (D)",down, LENGTHUNIT["metre",1]], ID["EPSG",5706]]], METHOD["Vertical Offset", ID["EPSG",9616]], PARAMETER["Vertical Offset",-28, LENGTHUNIT["metre",1], ID["EPSG",8603]], OPERATIONACCURACY[0.0], USAGE[ SCOPE["Change of height to a different vertical reference surface."], AREA["Azerbaijan - offshore; Kazakhstan - offshore; Russian Federation - Caspian Sea; Turkmenistan - offshore."], BBOX[37.35,46.95,46.97,53.93]], ID["EPSG",5440], REMARK["The Baltic 1977 vertical reference surface is 28m above the Caspian vertical reference surface."]]
Definition: JSON
{ "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json", "type": "Transformation", "name": "Baltic 1977 depth to Caspian depth (1)", "source_crs": { "type": "VerticalCRS", "name": "Baltic 1977 depth", "datum": { "type": "VerticalReferenceFrame", "name": "Baltic 1977" }, "coordinate_system": { "subtype": "vertical", "axis": [ { "name": "Depth", "abbreviation": "D", "direction": "down", "unit": "metre" } ] }, "id": { "authority": "EPSG", "code": 5612 } }, "target_crs": { "type": "VerticalCRS", "name": "Caspian depth", "datum": { "type": "VerticalReferenceFrame", "name": "Caspian Sea" }, "coordinate_system": { "subtype": "vertical", "axis": [ { "name": "Depth", "abbreviation": "D", "direction": "down", "unit": "metre" } ] }, "id": { "authority": "EPSG", "code": 5706 } }, "method": { "name": "Vertical Offset", "id": { "authority": "EPSG", "code": 9616 } }, "parameters": [ { "name": "Vertical Offset", "value": -28, "unit": "metre", "id": { "authority": "EPSG", "code": 8603 } } ], "accuracy": "0.0", "scope": "Change of height to a different vertical reference surface.", "area": "Azerbaijan - offshore; Kazakhstan - offshore; Russian Federation - Caspian Sea; Turkmenistan - offshore.", "bbox": { "south_latitude": 37.35, "west_longitude": 46.95, "north_latitude": 46.97, "east_longitude": 53.93 }, "id": { "authority": "EPSG", "code": 5440 }, "remarks": "The Baltic 1977 vertical reference surface is 28m above the Caspian vertical reference surface." }
Definition: Mapnik
<?xml version="1.0" encoding="utf-8"?> <Map srs="+proj=geogoffset +dh=-28"> <Layer srs="+proj=geogoffset +dh=-28"> </Layer> </Map>