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Drape Geoid Heights Over Topography

This example shows the figure of the Earth (the geoid data set) draped on topographic relief (the topo data set). The geoid data is shown as an attribute (using a color scale) rather than being depicted as a 3-D surface itself. The two data sets are both 1-by-1-degree meshes sharing a common origin.

Load the topographic ( `topo` ) and geoid regular data grids.

```load topo load geoid```

Create a map axes using a Gall stereographic cylindrical projection (a perspective projection). Use `meshm` to plot a colored display of the geoid's variations, but specify `topo` as the final argument, to give each `geoid` grid cell the height (z value) of the corresponding `topo` grid cell. Low geoid heights are shown as blue, high ones as red.

```axesm gstereo; meshm(geoid,geoidrefvec,size(geoid),topo)```

For reference, plot the world coastlines in black, raise their elevation to 1000 meters (high enough to clear the surface in their vicinity), and expand the map to fill the frame.

```load coastlines plotm(coastlat,coastlon,'k') zdatam(handlem('allline'),1000) tightmap```

Due to the vertical view and lack of lighting, the topographic relief is not visible, but it is part of the figure's surface data. Bring it out by exaggerating relief greatly, and then setting a view from the south-southeast.

```daspectm('m',200); tightmap view(20,35)```

Remove the bounding box, shine a light on the surface (using the default position, offset to the right of the viewpoint), and render again with Gouraud shading.

```ax = gca; ax.Box = 'off'; camlight; lighting Gouraud```

Finally, set the perspective to converge slightly (the default perspective is orthographic). Notice that the geoid mirrors the topography of the major mountain chains such as the Andes, the Himalayas, and the Mid-Atlantic Ridge. You can also see that large areas of high or low geoid heights are not simply a result of topography.

`ax.Projection = 'perspective';`

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