CRJ200 First Officer receives a low altitude alert from TLH Tower during GPS 18 approach. Incorrect minimums of 520 ft are being used when aircraft capabilities require minimums of 800 ft.

Narrative

We were conducting a scheduled Part 121 flight to TLH. I was the Pilot Monitoring and the Captain was the Pilot Flying this particular leg. While enroute with approximately 35 minutes of flight time remaining; the Captain and I began preparing for the approach to TLH. Weather conditions in TLH were IMC at the time and required that an instrument approach be flown. I tuned in the ATIS frequency and attempted to retrieve it; but it turned out to be inaudible from our location due to strong interference from another station. Unable to determine the approach in use from the ATIS; we retrieved the latest TLH METAR via ACARS to determine the latest weather conditions and plan an approach accordingly. The METAR reported winds 160 degrees at 11 KTS; visibility 2 1/2 miles; and an overcast ceiling at 400 FT. Based on the approaches available for TLH; we decided that the ILS 27 would be most suitable. Once I finished programming the FMS for that approach; the Captain conducted an approach briefing. Our focus then shifted to navigating around a line of thunderstorms that remained between us and TLH. Once we were past the line of weather; we were nearing TLH Approach airspace. I tried once again to retrieve the latest ATIS; which was now audible from our location. The ATIS indicated that Runway 27 had been closed; which we had no prior NOTAM indicating. We later found out that the closure was due to an aircraft incident on Runway 27. Due to the unavailability of Runway 27; the ATIS indicated that the RNAV/GPS 18 approach was in use. The Captain and I began preparing and briefing the RNAV/GPS 18 approach. Due to the last-minute nature of the information we received about Runway 27's closure; we did not have as much time available to prepare for this approach as we did for the ILS 27. Nevertheless; I attempted to be as thorough as possible in helping prepare for the approach and in programming the FMS. Referencing our approach plate; we briefed and pre-selected an MDA of 520 FT for the RNAV/GPS 18. Soon thereafter; TLH Approach cleared us direct to the CUPAM IAF and cleared us for the RNAV/GPS approach. The approach began normally; with the Captain descending to the published stepdown altitudes as appropriate. After reaching the JAPMA FAF; he initiated a descent to an MDA of 520 FT. At approximately 600 FT; we broke out of the clouds and I called out 'runway in sight.' At about the same time; TLH Tower gave us a low altitude alert and advised us to check our altimeter setting. I cross-referenced both my altimeter setting and the Captain's with the setting provided by the Tower. All 3 were identical. I noticed that the Captain was still above our selected minimums; and had begun a slight climb in response to the Tower's alert. We soon reached the VDP; and with the runway still in sight; the Captain began a descent for landing and landed normally. My initial impression was that the low altitude alert may have been erroneous; as our altimeters had been set correctly and the Captain was flying above the selected MDA. It was not until the following day that we discovered our mistake. The Captain; trying to determine why we had received a low altitude alert; went back and reviewed the RNAV/GPS 18 approach plate a second time. The approach plate that we used for the approach contains 5 different columns of minimums. 2 of those 5 columns have a large bold heading that reads 'RNAV GPS.' Underneath those 2 headings; in small; non-bold print; are the words 'VNAV' in the first column and 'LNAV' in the second. The MDA in the VNAV column is listed as 520. In the LNAV column; the minimums are listed as 'Use circling.' The circling minimums column lists the minimums as an MDA of 800 and visibility requirement of 2.25 miles. Based on this; our MDA for the approach should have been 800; not the 520 that we had used. Clearly we made a mistake that could have had a much more severe outcome than what we experienced. I believe there are many factors that ultimately led to us making this mistake. First and foremost; the last-minute change from the ILS 27 that we had earlier prepared to the RNAV/GPS 18 did not allow for as much time to prepare for the approach as we would have liked. I believe that we both allowed ourselves to become rushed as we set up the new approach; which we should not have done. Fatigue and workload may have also been a factor; as we had been navigating around severe weather for much of the duty day. We were using a type of approach (RNAV/GPS) that is used only on rare occasions in our operations. In addition; our overnight prior to the start of the duty day was a reduced rest overnight. I believe that the format of the commercial chart 1 approach plate we were using also contributed. Approximately 1 year ago; our airline decided to discontinue using Company B approach plates and transition to the Company A RouteManual product. The decision appeared to be motivated by lower cost more than anything else. While commercial chart 2 charts are the industry standard and have been used by virtually all of the airline's 1300+ pilots for most of their aviation careers; few Pilots were familiar with the commercial chart 1. The company provided only a brief web-based training system for pilots to complete prior to using commercial chart 1 charts for company flight. I believe that this apparent weakness in format is still no excuse for our mistake. We still should have been able to interpret the information from the Company A chart correctly. At the same time; however; I believe that the likelihood of our mistake would have been reduced significantly had the commercial chart 1 chart been as clear and concise as either the commercial chart 2 or Government charts. In the future; I will make a conscious effort to be much more vigilant when reviewing the commercial chart 1 charts. In addition; when making last-minute approach changes such as the one we made; I will ensure that we take as much time as we need to review and prepare the approach more thoroughly -- even if we need to request vectors or additional time from ATC to do so. Furthermore; if there is any degree of doubt during an approach situation such as this again; I will call for a missed approach so that we can double-check the approach preparation once again.