ACR WDB DESCENDED TO NEXT ALT AFTER RECEIVING VISUAL APCH CLRNC. APCH CTL INSTRUCTED THEM TO MAINTAIN THE HIGHER ALT.

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ON APCH TO LGA; ATIS WAS GIVING VOR 'C' TO RWY 22 AND WE SET UP ACCORDINGLY. APCHING NY HARBOR WE WERE ADVISED RWY 13 WAS AVAILABLE. WE REPLIED THAT WOULD BE ACCEPTABLE AND WERE GIVEN A HDG AND TOLD TO MAINTAIN 3000' AND TO RPT THE STATUE IN SIGHT. I REPLIED WE HAD THE STATUE AND RIVER IN SIGHT. APCH THEN CLRD US DIRECT TO THE STATUE AND CLRED US FOR A RIVER VISUAL APCH TO RWY 13 TO MAINTAIN 2000' UNTIL NORTH OF CENTRAL PARK. WE READ BACK THE CLRNC AND DSNDED TO 2000' WHICH CONCURRED WITH THE ONLY ALT GIVEN ON COMMERCIAL PLATE 19-2 FOR THE RIVER VISUAL APCH TO RWY 13. APCH THEN QUERIED OUR ALT AND SAID WE SHOULD BE AT 3000'. WE CLBED BACK TO 3000'. THE RIVER VISUAL APCH PLATE DOES NOT DEPICT A 3000' LIMITATION. IF ATC REQUIRES AN ALT TO A CERTAIN POS IT SHOULD BE ON THE APCH AND DEFINED BY A RADIAL OFF LGA. CLRNC FOR AN APCH INCLUDES CLRNC TO THE NEXT ALT ONCE ESTABLISHED ON COURSE. IN THIS INSTANCE; ESTABLISHED ON THE HUDSON RIVER THE PLATE SAYS 'MAINTAIN 2000' OR ABOVE AS LONG AS POSSIBLE.' THE APCH PLATE IS AMBIGUOUS AND SHOULD BE REDESIGNED TO SATISFY ANY ALT AND GEOGRAPHICAL REQUIREMENTS OF ATC WITH REFERENCE TO NAV AIDS AND RADIALS TO ELIMINATE CONFUSION. SINCE THERE WAS NO TFC RPTED OR SIGHTED BY US NO EVASIVE ACTIONS WERE REQUIRED. HOWEVER; BY HAVING TO MAINTAIN 3000' INSTEAD OF 2000' AS DEPICTED ON THE APCH PLATE; AN UNCOMFORTABLY STEEP APCH WAS REQUIRED TO LAND. THESE AMBIGUITIES SHOULD BE ELIMINATED BY REDESIGNING THE APCH PLATE TO REFLECT ANY ATC REQUIREMENTS IN DEFINITIVE TERMS.