I do not have information about the current digital grid, but here is info that I developed in the early 1990's just when they were creating the digital maps (in the san francisco bay area). Prior to the mid-1990's (exact date unknown), all thomas brother maps were on hand created bases. The base maps had significant, apparently random, stretch in both X and Y directions (rubber sheeting). The original TB grid was based on the california state plan cooridinate system. The grid was 2000 feet (north-south) by 3000 feet (east west). If you look at the 1994 maps you will see the cal. coord. numbers listed on the maps. The use of a rubber sheeted base map with a superimposed fixed sized grid created interesting problems. For example, look 1994 alameda county maps, pages 59-60. Measure the distance between the toll plaza and Hesperian Blvd on the TB map (using their grid). Then measure the same distance on a USGS 7-1/2 minute quad. They will be quite difference. More importantly, compare the two maps [look at the relative relationships between objects, and measure distances] in this area and you will see the random stretch [rubber sheeting] on the TB map. At the time, I was just beginning a project to locate wells and began to use the TB maps to determine the calif. coords of the wells. I did some double checking with the USGS maps and quickly discovered the problem ( I had to re-located about 400 wells). Circa 1996, they came out with the digital base maps. If you look at page ix in a 1997 map book of alameda county, you will see the conversion map between the old and new grids. The odd ball shape of the old maps is quite evident. What I do not know is the coord system of the new grid. Is it still cal. coord. based or did they go to a lat long base? You can do two things. They publish a digital version of their maps (a CD) that will generate coords. Use the CD to detemine coords of various grid intersection and you can back out the grid framework. Or, probably much easier, call them (1-800 899-maps?) and as to speak to the cartagraphic section. They may provide detailed information on the grid itself at no charge. They charge lots if you want digitial information for address location. good luck - if you find further info, please post s. figuers original message Has anyone seen, developed, heard of etc. a grid that matches that of the 'Thomas Guide Grid' by Thomas guide maps. Or would anyone have ideas on developing a grid that matches the existing Thomas guide grid that covers the San Francisco Bay area. The grid would need to be 1.4" = .5 mile (great scale) Thanks in advance. Isaac Wiley BHC Overland Park, Kansas [EMAIL PROTECTED] _______________________________________________________________________ List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, send e-mail to [EMAIL PROTECTED] and put "unsubscribe MapInfo-L" in the message body.
