# HydroGeoSphere/Grid Projection

Figure 5.5 illustrates the degree of distortion that can occur (which is especially severe at the poles) when a spherical geographic coordinate system, such as longitude and latitude, is mapped directly into a planar 2-D coordinate system. The following command can be used to correct the distortion which occurs when modelling large continental scale problems.

## Project gridEdit

**projt**The projection type to be applied. Currently, the only valid inputs for**projt**are albers equal-area or lambert azimuthal equal-area, which apply the Albers conical or Lambert azimuthal equal-area projections respectively. For the Albers projection, the following additional data are required:

- (a)
**lon0, lat0**The longitude and latitude for the origin of the projection. - (b)
**lat1, lat2**The latitude of the standard parallels. - (c)
**datum**The datum used for the projection. Acceptable values for the variable**datum**are:- 1 Sphere.
- 2 Ellipsoid (Clarke 1866).

- For the Lambert projection, the following additional data are required:
- (a)
**lon0, lat0**The longitude and latitude for the origin of the projection. - (b)
**datum**The datum used for the projection. Acceptable values for the variable**datum**are:- 1 Sphere.

- (a)

Transforms the current mesh from a spherical system into a planar 2-D coordinate system. It must be inserted just before the End grid generation command.

The following example uses the Project Grid instruction:

```
generate blocks interactive
grade x
-150 -50 5 1 5
grade y
40 85 5 1 5
grade z
0 10 10 1 10
end
project grid
albers equal-area
-100 60 ! projection origin
55 70 ! standard parallels
1 ! 1=sphere, 2=ellipsoid
end
```

The grid produced by the Generate blocks interactive instruction is shown on Figure 5.5 and once projected, the grid looks like the one on Figure 5.6:

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## Project 2d gridEdit

The input data for this instruction is identical to that required for the Project grid instruction described above.

Transforms the current mesh from a planar 2-D coordinate system into a spherical system. It must be inserted just before the End grid generation command.

**• • •**

## Unproject 2d gridEdit

The input data for this instruction is identical to that required for the Project 2d grid instruction described above.

Transforms the current mesh from a spherical coordinate system into a planar 2-D system. It must be inserted just before the End grid generation command.

**• • •**