[gdal-dev] requesting tiles according to my dteds data c++

[len lee]

hello;
i downloaded dted data dt0 from raster roam website and there are 3 folders
e055, e056 and e057 and within each folder there are 3 dted files and i
guess each dted file here represent a tile within my terrain. now what am
trying to do is requesting the terrain tiles according to my dted data ..
any idea how can i do that with gdal lib?
i tried this:
poDataset = (GDALDataset *) GDALOpen( "
http://tile.openstreetmap.org/15/23687/15190.png", GA_ReadOnly );

to request a specific tile from openstreetmap server but as you can see i
need to know the exact coordinate system for that tile ...

please all what i need is to understand the process of requesting the tiles
according to my dteds files;

i will appreciate any help;
thanks in advance;
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