By Fred Limp, director, Center for Advanced
Spatial Technologies (www.cast.uark.edu),
University of Arkansas, Fayetteville, Ark.
A dirty little secret is becoming widespread as
increasingly high-resolution imagery becomes more common. The vector
data in many geospatial systems don’t line up! Many groups who have
acquired high-resolution imagery have been, at best, disappointed when
they overlay street centerlines or even parcels on their newly acquired
data. LineWorks, the first commercial software release from Pixxures
Inc., was developed to address this complex situation. And for a product
that tackles such a complex process, it’s surprisingly easy to use and
apply.
Why the Discrepancies?
Vector-alignment problems exist for many reasons. Sometimes it’s a
simple case of doing something dumb, but fixable, like getting the
imagery in one datum and the vectors in another. In other situations the
vector data were derived from lower quality sources. A common example
might be overlaying U.S. Geological Survey transportation data derived
from 1:24,000-scale maps on new 1-foot-pixel photography. Vector data
often are developed from many sources with different quality levels.
And then there are parcels. With few exceptions, the U.S. parcel
database is based on 18th-century measuring principles and legal rules
that are the antithesis of modern mapping. This isn’t a criticism of
today’s surveying community, but rather the maddening archaic legal
structures under which they often labor.
A parcel boundary is commonly defined by its relationship to some point
of beginning (POB) by meets and bounds (“north 23.5 degrees 350 feet
thence east …”) or by aliquot parts (e.g., the northern quarter of the
southern quarter of Section 1 Township 1 Range 2). If the POB or section
corner’s location is in question—and many are—then all the measurements
that follow are in question, and all the parcel
boundaries that reflect a specific POB will differ slightly from a POB that uses another
location. Knowing the accurate location of a POB can provide a surveying
company with a commercial advantage, so there can be good reasons to
avoid sharing such information. There are also simple errors in
surveying, as well as many chances for error when entering the survey
information into a geographic information system (GIS). Until recently,
many mainstream GIS packages didn’t support the complex data structures
required by survey data.
For all of these and other reasons, digital parcel maps almost
never “line up” with highly detailed, accurate digital imagery.
The question when faced with these obvious discrepancies is “What
to do?” The right answer is a case-by-case “It depends,” but a
common and useful approach in many situations is to adjust the
vector data to the imagery, which is a complex process. Because
parcel data are derived from multiple POBs with multiple error
sources, it’s impossible to simply “warp” the vector data to the
imagery as one might do with two images. No uniform mathematical
transformation can be applied. In addition, there are key
relationships that must be maintained. If a lot corner is
adjusted, all the parcel boundaries that come from that corner
must be moved and the topological relationships maintained; no
parcel polygon can be broken apart, for example, or relocated to
overlap another.
Using LineWorks
Installing LineWorks is simple and straightforward. Once
installed, the machine code was sent to Pixxures and the
appropriate licensing code came back. The version I reviewed was a
single user “desktop” version in which data, processing and
display are on the same machine. There are also Enterprise and
Enterprise Internet solutions in which only the graphic display is
local; all data and processing is performed on a server.
Once installed, a user simply selects the appropriate vector and
raster data. LineWorks supports ESRI Inc. shapefiles and E00
vector formats, as well as PostScript files. In the raster domain,
the software imports JPEG2, TIF, GeoTIF, ECW, Zeiss (INP) and
Vision files. The actual work flow involves creating “links”
between the current source vector and the place where the specific
vector element “should” be on the image. The process simply
involves clicking on the source element and then clicking where it
should be. A “link” line is drawn on the display. The user can set
the source snapping distance. The user can also specify whether
the “adjusted” (LineWorks uses the term conflated) vector is
displayed, so the effect of the move can be seen immediately. The
user also can specify whether to display nodes (end points) and/or
vertices in the source vector.
It’s also possible to create bookmarks, which are specific areas.
Once a bookmark is selected, LineWorks will quickly locate the
view and the data within the selected area. This is useful for
quality control and for areas that require repeat visits. In
addition to the capability to snap to a source vector, it’s
possible to define a link that is not snapped to any vector
element but can be viewed as linked to the general vector theme at
the link’s starting point. It’s almost as if the link is placed to
a point on a transparent sheet on which the vectors are drawn. At
first this might seem counterintuitive, but such functionality can
be particularly useful with geometric features, such as a circular
object like a roundabout. In this case, the “center” of the
polygon (which has no specific element) can be linked to the
center of the feature on the image.
Useful Tools
Once created, links can be edited by selecting the edit link
option and moving the end points with the mouse, or a link can be
deleted. There are useful link undo and redo buttons, as the best
link placement can frequently be a case of trial and error. A
particularly clever tool is the guide line, which allows the user
to draw a reference or guide line on the imagery that can serve as
the basis for linking. For example, consider a situation in which
two fence lines indicate a parcel boundary but don’t extend to the
property corner. Two guide lines can be drawn to effectively
extend each fence line, and where they cross is the corner. That
point can be used to position the end of the link. Guide lines
also can be edited. The “copy link” function effectively allows
the user to apply a similar adjustment defined by an initial link
to different locations. All links are stored in a separate link
file without modifying the original data. When all links have been
created, the user runs the conflation module. Input fields are the
original shapefile, and the links file and output is an adjusted
(conflated) shapefile. If desired, the user also can output a
topology error report. A similar sequence is applied to E00 files.
The precise method Pixxures’ programmers used to perform the fit
is proprietary, and there is no “error assessment” or a measure of
“goodness of fit.” It’s difficult to imagine how such a measure
might be calculated. As a result, the quality of the fit must be
assessed visually, and this must be done largely on screen.
Hard-copy output capabilities are limited, though new hard-copy
capabilities are forthcoming.
The software comes with a 21-page, ring-bound Users Guide and a
95-page tutorial manual. The guide provides menu options and
alternatives, and the tutorial provides a good introduction to
LineWorks, along with a tutorial data set.
In today’s world of high-resolution imagery, and with vector files
of varying qualities, LineWorks provides an easy-to-use solution
to fit one to the other. If you face this dilemma, contact
Pixxures for a LineWorks evaluation copy to see if it meets your
needs.
