1. The Projection Aspects of Digitising Globes
Mátyás Gede
Department of Cartography and Geoinformatics
Eötvös Loránd University, Budapest
The Projection Aspects
of Digitising Globes

2. Contents The Virtual Globes Museum Processing globe prints
Processing photographs
Building 3D models
Creating a facsimile globe
Discussion

3. The Virtual Globes Museum
On-line exhibition of 3D globe models related to Hungary
Source materials: original unmounted gores or photo serieses of the globes

4. Possible Inputs

5. Possible Outputs

6. Processing Prints - georeferencing - projection transformation
GCPs: grid intersections Projection: Cassini (alternatives: Transverse Mercator; Polyconic)
- projection transformation
to Plate-Carée
- assembling the pieces

7. Processing Photos
- Georeferencing (projection: Near-Side Vertical Perspective)
- Projection transformation (to Plate-Carée)
- Assembling the pieces

8. Georeferencing photos
GCPs: grid intersections
Projection: Vertical Near-Side Perspective
- There are parameters missing:
- Centre of projection
- Perspective height
Solution: the Globe Photo Parameter Calculator
- a utility developed by the author
- based on Nelder-Mead Downhill Simplex Method
- finds the best parameter set to achieve minimal errors at the control points

9. Determining projection parameters
The parameters to be determinded:
(φ,λ) – center of the projection
h – height of perspecive point
Additional parameters to place
the projection on the photo:
(Δx; Δy) – offset
δ – rotation
M – scale (pixels/km)
An implementation of the downhill simplex method finds the appropriate parameter set to minimize the difference of the given and the calculated pixel coordinates at the GCPs.

10. Georeferencing Photos: the workflow
1st step: Setting and saving GCPs
2nd step: Calculating parameters
3rd step: Applying the calculated projection settings

11. Virtual Globes
VRML model:

12. Virtuális gömbök
KML model in Google Earth:

13. Creating a New, Virtual Globe
Direct creation of the texture files for the VRML model
Examples: globes of the Mars, Moon an Venus by Henrik Hargitai -
Further research is needed to determine how to take the distortions in count while editing the map

14. Facsimile Globe Production
- Creating gores
Platte-Carrée segments are transformed to Cassini projection (transverse form of Equirectangular projection)

15. Facsimile Globe Production
- Mounting

16. Discussion Accuracy of georeferencing a photo depends on
the distance of the globe centre from the optical axis
the shape of the globe (it is never a perfect sphere)
imperfectly mounted gores
Appearance of Moirë effect (interference between the printing rasters and the sampling grid)
Advisable to apply a Moirë filter on the scanned image
Special non-spherical globes – no problem in VRML
Polar areas in Google Earth

17. Further information:
Virtual Globes Museum
Mátyás GEDE

18. Thanks for your attention!