GA SMT MADE APPROX 3 NM ERROR WHEN GIVING POSITION RPT TO MDT TWR.

Narrative

I DEPARTED BWI ARPT FOR A FLT TO NEW ENGLAND. UPON DEPARTING; BWI TWR ADVISED I CONTACT DEP AND I WAS GIVEN A HEADING TO FOLLOW. AFTER SEVERAL MINUTES INTO FLT BWI DEP ADVISED I COULD ASSUME MY OWN NAVIGATION; I WAS 10 MILES NE OF BWI AND THAT I MAY ENTER THE MARTIN STATE (MTN) ARPT TFC AREA AND I SHOULD CONTACT MARTIN TWR FOR TRANSITION ON 121.3. I CONTACTED MARTIN TWR; REPORTED MY POSITION (FROM THE INFORMATION PROVIDED MY BWI DEP I.E. 10 MILES NE OF BWI); ALT; AND ROUTE OF FLT. AFTER A SLIGHT DELAY; THE TWR ACKNOWLEDGED AND REQUESTED I REPORT 2 NM FROM THE FIELD. THIS WAS A PROBLEM BECAUSE MY DME WAS OUT OF SERVICE (MTN HAS A VOR ON THE FIELD); AND NOT BEING FAMILIAR WITH THE AREA I HAD NOT PINPOINTED MY RADAR POSITION TO A GND POSITION ON THE WASHINGTON TCA MAP. HENCE THERE WAS A DELAY IN MY ATTEMPTING TO DETERMINE '2 NM FROM THE FIELD.' IN THE END I WASN'T SURE AND REPORTED 2 NM TO WHAT 'APPEARED' TO BE 2 NM AWAY. LATER WHEN I HAD MORE TIME; I USED A RULER TO MEASURE THE DISTANCE TO THE FIELD FROM THE LAND MARKS I SAW ON THE GND. IN REALITY I WAS 4.5 NM FORM THE FIELD. CORRECTIVE ACTION: I'D LIKE TO SEE ONE OR BOTH OF THE THESE DONE:WHEN WORKING AN ATC FACILITY AND THAT FACILITY WILL ROUTE YOU THROUGH A ATA EITHER:A) ATC OBTAIN A CLRNC ON YOUR BEHALF TO REDUCE FREQ SWITCHING; B) IF ATC MUST HAND YOU OFF; THEN INSTEAD OF GIVING A POSITION IN RELATION TO THE DEPARTING FAC ALSO GIVE A POSITION THAT IS IN RELATION IN THE ATA. EXAMPLE ACFT XXXX 10 NE OF BWI; CONTACT MARTIN STATE TWR FOR ADVISORIES FOR TRANSITIONING THE MARTIN ATA; YOU ARE 15 MILES SW OF MARTIN STATE NOW (THIS PIECE OF INFORMATION WOULD HAVE BEEN INVALUABLE TO ME). PUT A CIRCLE AROUND ALL ATA'S AS IS NOW DONE FOR ARSA'S TCA'S. IT WOULD MAKE CORRELATING GND POSITION TO ARPT POSITION MUCH EASIER.