Global Mapper v25.0

Polyconic Projection

smile
smile Global Mapper UserTrusted User
edited September 2007 in Technical Support
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

Comments

  • global_mapper
    global_mapper Administrator
    edited September 2007
    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
  • smile
    smile Global Mapper User Trusted User
    edited September 2007
    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
  • global_mapper
    global_mapper Administrator
    edited September 2007
    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
  • smile
    smile Global Mapper User Trusted User
    edited September 2007
    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
  • global_mapper
    global_mapper Administrator
    edited September 2007
    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
  • smile
    smile Global Mapper User Trusted User
    edited September 2007
    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