Thaks to all those wo have replied. In short the sensitive answer is :
If there are not many neighbourhood do it by hand. If you have MI6.0 use the copy to comestic like convexhull to join the selected blocks, then follow corrections by hand. All the other answers are appreciated but always have to deal with the "street space" problem. ie the public space between street centerline and line of edification. It has been done by hand for some 3000 blocks, uf !!!!. Following the thread it happens to me that this is a classical space where a good (non existent here) mix between raster and vector gis will be wonderfull. Paint block by different colors based on neighborhood class. Put street centerlines. Freeze the scene and pass it to raster (geotiff). Paint bucket "street space" (between block and street centeline) with same color that neighbourhood, select with magic wand, vectorice by nbhood color, pass it back to vector gis, or something in this lines. Juanse temuko-Chile --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.441 / Virus Database: 247 - Release Date: 10/01/03
