# 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 grid

1. 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.

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
-150 -50 5 1 5
40 85 5 1 5
0 10 10 1 10
end

project grid
albers{{typo help inline|reason=similar to alberts|date=September 2022}} 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 grid

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.

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## Unproject 2d grid

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.

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