March 1, 2011

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Mapping designations and surveys undertaken before 2010: We have scanned and georeferenced all 1500 USGS maps generated by the survey, National Register, and environmental review programs since the 1970s.  The USGS maps are the starting point for mapping properties and districts designated before 2010 and surveyed sites in rural areas recorded before 2010. Mapping by USGS is supplemented with high-resolution NC OneMap aerials, Google Street View, and Bing Birdseye where available, compared against ground photos.

All National Register boundaries have been drawn from a comparison of the official map in the nomination file with tax parcel layers and aerial photography in GIS. Most boundaries for non-archaeological properties and districts are based all or in part on parcels. Some early nominations (generally before 1980)  do not have clearly defined or delineated boundaries, and it is not possible to draw a meaningful boundary.

Study List boundaries are at best a suggestion of what the boundary would be if a property or district were nominated to the National Register.  Most individual Study List entries do not have suggested boundaries. Study List district boundaries are by definition tentative, or simply indicate that there is a potential district within the area shown.

Determination of Eligibility (DOE) boundaries are sometimes based on parcel maps included in DOE reports, and at other times simply a suggestion of the DOE area. We are refining DOE boundaries as we work back through old reports to identify more precise boundaries.

Errors and lost properties: Surveyors sometimes make errors when mapping a property, and sometimes USGS maps themselves contain errors.  Many of our surveys are 30 or more years old, and survey updates reveal that as many as 35% of places recorded in older surveys have been lost.  When we can confirm that a property has been lost (as by a report from the field or by a comparison of aerials made in different years), the property is coded to show it is gone with a black dot within its symbol, and "(Gone)" is appended to the property name.  If no historic building appears to be present at a site marked on the USGS map but we cannot confirm whether it may have been mapped in error, the name is appended with "(Gone?)", but the symbology is not changed.

Mapping designations and surveys since 2010We ceased marking paper USGS maps with new designations in early 2010 when we were able to provide a GIS connection for visitors to our map room, and marking paper maps became redundant.  Newly designated sites are mapped in GIS from maps and other data in NR nominations, Study List applications, and DOE reports, using tax parcel layers, aerials, and other sources to confirm building locations and boundaries.

New surveys are mapped as follows, often using a combination of techniques:
(1) Joining PIN numbers entered in the survey database with county tax parcel layers to map the whole property, followed by setting a point on the principal surveyed feature. For urban and historic district surveys where parcels are generally small, the point may be set by calculating the centroid. Rural surveys with large parcels require additional sources to determine the location of the principal feature when it is not obvious.
(2) Scanning and georeferencing newly marked USGS or other maps and setting points adjusted with aerials and Street View
(3) Geocoding street addresses (urban surveys only), adjusted with aerials and Street View.

GPS units: Our limited experiences with GPS units and GPS cameras used in surveys have so far yielded inconsistent results. Better equipment and training may improve results with GPS devices in future surveys.