Getting the national dex is very easy in Diamond/Pearl compared to the other games because you only have to see all 150 of the sinnoh dex and it is possible to see all 150 by fighting all the trainers including the elite four and speaking to the old lady in the big house in Celestic town after encountering Dialga/Palkia and she'll show you Palkia/Dialga. You'll also have to defeat the elite four then you talk to Rowan and you'll get the nat dex and the pokeradar
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