Mapping Toolbox

Switching Between Projections

When switching between projections using setm or the graphical interfaces, you may need to change some of the map axes properties for proper appearance. Settings that are suitable for one projection may not be appropriate for another. Some projections have default properties that define that particular projection and may not be altered; for example, the Balthasart Cylindrical projection is defined to have standard parallels (MapParallels) at 50°. Other projections have default properties that are initially set for proper world display; for example, the Mercator projection limits the latitude range to ±86° to avoid "blowing up" at the poles.

Although similar projections may share the same set of properties (Miller Cylindrical and Plate Carree Cylindrical), others may be drastically different (Polyconic and Stereographic azimuthal). The classification of the map projections is often a good indicator of whether changes need to be made. For instance, switching from a cylindrical to an azimuthal projection requires a few modifications:

• axesm mercator
  framem on; gridm on; mlabel on; plabel on
  setm(gca,'LabelFormat','signed')
  
  
  

To display the default map and frame latitude limits for the Mercator projection, type the following:

• [getm(gca,'MapLatLimit'); getm(gca,'FLatLimit')]
  ans =
  
  -86    86
  
  -86    86
  

Both the frame and map latitude limits are set to ±86° for the Mercator projection to maintain a safe distance from the singularity at the poles.

Now switch the projection to an Orthographic azimuthal:

• setm(gca,'MapProjection','ortho')
  

What happened to the map frame and labels? If you recall, the frame latitude limits have not been changed and still correspond to the default values for a Mercator projection, as do all the other properties. Only those properties that must have particular values are updated for the current projection.

• getm(gca,'FLatLimit')
  ans =
  
  -86    86
  

You must manually reset the frame and map limits to appropriate values for an Orthographic projection so that the circular frame is displayed. If you don't know the default or appropriate values, provide an empty matrix for any of the property values:

• setm(gca,'FLatLimit',[],'MapLatLimit',[])
  [getm(gca,'MapLatLimit'); getm(gca,'FLatLimit')]
  ans =
  
  -90    90
  
  -Inf 89
  

You will also need to manually specify the locations of the meridian and parallel labels:

• setm(gca,'MLabelParallel',0,'PLabelMeridian',-90)
  

Now the map is displayed correctly, with the frame:

Default property values can be accessed by the use of empty matrices, as shown in the last example, and the properties can be reset according to the "new" values. The entire set of properties can be reset to their default values by pressing the Reset button on the axesmui GUI.

For complete descriptions of all map axes properties, consult the axesm reference page in the "Mapping Reference" chapter of the online Mapping Toolbox reference documentation. For more information on the use of axesmui, also refer to the axesm, axesmui reference page in the "GUI Reference" chapter of the Mapping Toolbox online reference documentation.

Map and Frame Limits

Projected and Unprojected Objects