Polyconic Projection
smile
Global Mapper UserTrusted User
I have a paper map in which the projection is mentioned as " International Modified Polyconic Projection" and the grid is given in Lat Long.
I have taken Meridian as 100 in Global mapper ( East or West is not selectable) in Projection window. But at 80 Merdian my map seems to match the grid.
If you can please let me know if multiple meridians exist for this projection and I am not sure what exactly is modified from Polyconic ( American) . I also know that some of the projections are not particularly useful for navigation and I will have to reproject to standard projections and datum for navigational use/
If you can throw some light on my question ,it would be helpful.
Thanks
Smile
I have taken Meridian as 100 in Global mapper ( East or West is not selectable) in Projection window. But at 80 Merdian my map seems to match the grid.
If you can please let me know if multiple meridians exist for this projection and I am not sure what exactly is modified from Polyconic ( American) . I also know that some of the projections are not particularly useful for navigation and I will have to reproject to standard projections and datum for navigational use/
If you can throw some light on my question ,it would be helpful.
Thanks
Smile
Comments
-
It looks like there is a modified Polyconic projection like you describe that was used for some maps up through about 1962. The Polyconic projection supported in Global Mapper is the American Polyconic and is slightly different, but if you use that you should get pretty close if you use several control points when rectifying the data. The appropriate meridian to use will depend on your data, but just picking a value near the center of the map should work pretty well.
Note that to specify East longitudes, just use positive values, while West longitudes are negative.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
Yes. I also feel the same. The longitude in the map which is truly vertical or the centre longitude of the map which is near vertical should solve the problem. I am not sure of the difference it makes between American and the other, but I always make it a point to check the distance with the GM tool from the expanding scale given in the map and if they match, it should work fine. I hope GM distance tool measures the Ellipsoid distance(?)
Thanks
Smile -
Yes, the Measure Tool in Global Mapper always calculates the great circle distance along the surface of the ellipsoid if possible.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
After several trials, I would like to inform that the map is a modified polyconic projection only and polyconic (American) was not comaptible. It also appears most of the maps in this part seem to be in this projection only . From Googling, I understand that Manifold seem to support " Modified Polyconic Projection". I am not sure , if it is made for a specific country or it is the same as " International Map of the world" as you said of 1960s. It seems it has two meridians etc
Please have a look if it can be supported as most of the data is on this projection and Normal UTM/WGS84 data will take sometime to get published in this part of the world.
Thanks
Smile
P.S : If you would like to have a look at the Rasters, I can upload it to you on FTP -
If you can provide me with a sample I may be able to get the projection added. Our underlying projection library appears to already support at least the "International Map of the World" Polyconic, so it should not be too difficult to add.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
I shall do that in the morning tomorrow as I am on dialup now. In the meanwhile , please also have a look at the PM sent to you and the link.
I will be sending Rasters of different scales thro FTP
Thanks
Smile
Categories
- 12.7K All Categories
- 5.6K Features Discussion
- 342 Downloading Imagery
- 1.3K Elevation Data
- 380 Georeferencing Imagery Discussion
- 628 GM Script Language
- 53 User Scripts
- 113 GPS Features
- 414 Projection Questions
- 819 Raster Data
- 1.3K Vector Data
- 6.6K Support
- 177 Announcement and News
- 908 Bug Report
- 558 SDK
- 1.2K Suggestion Box
- 3.7K Technical Support
- 562 Other Discussion
- 129 GIS Data Sources
- 27 Global Mapper Showcase
- 233 How I use Global Mapper
- 107 Global Mapper Forum Website