PAMAP Breakline Information
A PAMAP White Paper
Prepared by the PAMAP Program
March 2009
The PAMAP Program provides four data products from the statewide LiDAR project: point
cloud, Digital Elevation Model (DEM) raster, contours (2-foot interval), and breaklines. At
project start, the breaklines were not considered a deliverable product, and were not specified as
a deliverable. They were designed for data production purposes. However, the Program saw
value in providing the breakline data for use in geospatial operations throughout the
Commonwealth. Consequently, the breaklines were requested as a deliverable and are distributed
with the other data products. Caution should be exercised when using the breakline data,
however.
The PAMAP breaklines may best be described as contour-enhancing lines, point cloud
classification aids, and orthophoto aids. The PAMAP breaklines were not designed or developed
to support engineering (e.g. H&H) or similar analysis or use. The breaklines may have to be
edited or supplemented to support such use.
Breaklines were collected photogrammetrically along most road edges, bridges/overpasses,
shorelines of open water bodies (lakes/ponds), shorelines of higher-order streams (double line
streams), railroads, and other miscellaneous features from PAMAP imagery. Some breaklines
from the 2007 project year were compiled from LiDAR stereo pairs. The breaklines were
initially digitized in 3D with an elevation value. The digitized elevation values for hard
breaklines (man-made) and bridges are retained. The elevation values assigned to the other
PAMAP breaklines are derived using elevations from the LiDAR points. Each breakline is
attributed for feature type (e.g. paved road); the feature types are documented in metadata. The
breaklines do not rigorously define those features that they were collected along. For example,
road edge breaklines may be captured near the edge of road, but not at exact edge. In addition,
the definition of paved and unpaved road edges is not verified or tested.
The hydro breaklines (lakes/ponds and double line streams) are digitized so that they may be
used to delineate areas of water. The right side of the line is the ‘water side’, so that one side of a
double-line feature is digitized in an upstream direction, with water on the right, and the other
line is digitized in a downstream direction, with water on the left. The hydro breaklines are
monotonic; monotonicity is the consistent flatness or decrease in elevation of a hydro breakline
in the direction of flow. The first ‘upstream’ elevation is always higher than the last downstream
elevation. However, variation is allowed, where a downstream vertex may have a higher z-value
than the upstream vertex, as long as this does not cause contouring problems. Such contouring
problems are discovered during visual inspection, and corrections to the breaklines may be
required. In certain cases, such as in areas of high terrain along the stream, or along an
undulating stream, corrected z-values may be applied to the line to better define contours.
The data along breaklines can be expected to be uncertain, especially when using two data
sources (LiDAR and imagery) from different years to identify the land/water interface. This is
the case with Luzerne County, where breaklines were defined with imagery from 2005 and
LiDAR from 2006. This is also the case with 11 of the 20 counties with 2007 LiDAR, where
breaklines are collected from 2005 imagery. This may also be the case for areas where the
imagery and LiDAR are collected in the same year, but at times of different stream or river
conditions. This is further complicated with vegetation overhang affecting both stereo digitizing
and penetration of LIDAR in these areas. This uncertainty may result in the water level of the
delineated feature to be higher than that of immediately adjacent land.
The breaklines are used in PAMAP data production in the following ways:
• The breaklines help maintain the position of linear ground features in orthophoto
production.
• The breaklines and model key LiDAR points are used to create a TIN, from which the
contours and DEM are produced.
• The breaklines around bridge features are used to reclassify the points within the bridge and
within 1.5’ of the breakline to class 1, Default. These reclassified points are removed from the
bare earth terrain model.
• The breaklines along road edges are used to reclassify the LIDAR points within 1.5’ of the
breakline to class 15, Road Edge. These reclassified LiDAR points are removed from the bare
earth terrain model. The road edge breaklines enhance the appearance of the contours crossing
the roads, and also maintain alignment of roads across adjacent terrain model blocks.
• To avoid edge match issues of contours across PAMAP tiles, not all road edge breaklines
continue to the tile edge.
• The breaklines along railroad beds are used to reclassify the LIDAR points within 1.5’ of
the breakline to class 15, Road Edge. These reclassified LiDAR points are removed from the
bare earth terrain model. The railroad breaklines enhance the appearance of the contours
crossing the railroad beds, and also maintain alignment of railroads across adjacent terrain
model blocks.
• The breaklines surrounding lakes/ponds and significant swamps/marshes are used to
discount returns off water (which are highly susceptible to error) and reclassify the points
within the features and within 1.5’ of the breakline to class 9, Hydro. These reclassified
LiDAR points are removed from the bare earth terrain model. These breaklines are used to
maintain alignment of water features across adjacent terrain model blocks. The breaklines
around lakes/ponds are of a constant elevation to ensure flat-water surfaces on open water.
• The double line drain breaklines are compiled along the banks of double line drains (greater
than 20’ wide), as well as along the edges of islands. These breaklines are used to discount
returns off water (which are highly susceptible to error) and reclassify the points within the
hydro features and within 1.5’ of the breakline to class 9, Hydro. These reclassified LiDAR
points are removed from the bare earth terrain model. The double line breaklines are used to
maintain alignment of hydrography across adjacent terrain model blocks. The double line
hydro breaklines enhance the appearance of contours crossing double line hydro.
Orthoimage and breaklines, Cumberland County, 2007