This　study　investigated　long-term　observations　from　three　pairs　of　closely　spaced,　continuously　operating　reference　stations　(CORS):　STVI　and　VITH　(2008-2014,　0.6　km　apart),　BYSP　and　PRHL　(2008-2014,　3.2　km　apart),　and　CRO1　and　VIKH　(1995-2014,　23　km　apart),　in　the　region　of　Puerto　Rico　and　Virgin　Islands.　The　Global　Positioning　System　(GPS)　antennas　of　STVI,　BYSP,　and　CRO1　are　mounted　on　ground-based　monuments.　The　GPS　antennas　of　VITH,　PRHL,　and　VIKH　are　mounted　on　building-based　monuments.　This　study　indicated　that　there　is　no　considerable　difference　between　building-based　and　ground-based　CORS　with　regard　to　the　precision　of　daily　positions　and　the　reliability　of　long-term　site　velocities.　The　amplitude　of　thermal　movements　associated　with　daily　temperature　changes　and　the　wind　drift　associated　with　air　pressure　change　from　a　2-story　concrete　building　fell　below　the　detectable　levels　of　the　present　high-precision　GPS　technique　determined　by　Precise　Point　Positioning　(PPP)　processing.　Further　analysis　indicated　that　(1)　building-based　CORS　can　retain　more　complete　GPS　observations　and　provide　a　more　stable　multipath　environment,　and　(2)　the　large　and　heavy　mass　of　the　building　and　its　foundation　can　decrease　the　amplitude　of　seasonal　signals　and　filter　certain　short-period　motions.　As　a　result,　the　building-based　CORS　have　the　potential　to　perform　better　than　the　ground-based　CORS　as　long　as　their　monuments　are　firmly　fixed　on　1-story　or　2-story　concrete　buildings　and　the　buildings　themselves　are　stable.