So what is a GIS portal? Many towns in southern New Hampshire maintain GIS data sites. These are great sources of information for home buyers (sellers also) in southern New Hampshire. The main screen shows a map of the town with all the physical features and roads. You can zoom in to any level. So say, you're interested in a house in Derry NH. You can go to the Derry NH GIS page and zoom into the address. You will see the parcel description and be able to view what is surrounding the property. You can add an aerial overlay which will actually show you the house on the lot and surrounding properties.
In addition, you can obtain a PDF property card, on the home you are interested in, which will give you the square footage and other pertinant details about the home. Back to the map, and you can view wetlands, flood zones, school districts, zoning and topography. Each town's system is a little different and offers a varying amount of information. So if you're a home buyer in New Hampshire, and you're looking for information about a certain house of parcel of land, be sure to check out the GIS sites maintained by the local cities and towns. Here are some convenient links:
WINDHAM NH GIS
DERRY NH GIS
LONDONDERRY NH GIS