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GGRS87 (EPSG:2100) is the projection system for Greece. But there is also another projection system which caters for a complex of islands (Kastellorizo, etc) which are located east form Greece (near Turkey). I have created this new projection with the following parameters (+proj=tmerc +lat_0=0 +lon_0=27 +k=0.9996 +x_0=500000 +y_0=0 +ellps=GRS80 +towgs84=-199.87,74.79,246.62,0,0,0,0 +units=m +no_defs).

I have digitized some polygons on these islands with the help of some orthophotos. However, these orthophotos were not on the correct location. They are inside the extent of the area for GGRS87 and not near Turkey. I saved this shapefile to the new projection. The map of the extent of the CRS for the shapefile shows that the area of the projection system is located on the west side of Africa. enter image description here On the contary, the map of the extent of the CRS for the whole project shows that the area of the projection system is near Turkey (which is the correct location). enter image description here

How is this possible? Has the fact that I did not determine the extent when I created the new projection played a role in it? In addition you can download the polygon shapefile here.

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  • defining extent should not be an issue. doesn't look like a coordinate issue. It looks like geographic coordinates taken as cartogrphic coordinates (you are near the 0,0). It is unclear to me where the problem come from. Could you provide the coordinates at each step ?
    – radouxju
    Commented May 3, 2018 at 10:22
  • Can you become more detailed? What coordinates do you want to provide?
    – Ilias Machairas
    Commented May 3, 2018 at 11:01
  • the coordinates of the orthophotos (e.g. upper left corner) and the coordinates of one vertex of one digitized polygon
    – radouxju
    Commented May 3, 2018 at 13:08
  • the coordinates of the bottom left corner of one orthophoto are 730000, 4003500.
    – Ilias Machairas
    Commented May 3, 2018 at 14:01
  • You can read the extent from the layer properties, metadata tab, properties window in QGIS. It seems the projection uses +lon_0=30 instead of 27. An offset of about 300m remains though.
    – AndreJ
    Commented May 3, 2018 at 17:10

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