NC Streambed Mapping Project
Issue Paper
ISSUE 8: Ground Cover and Base Data Characteristics for the Determination of
Horizontal Accuracy
Background
The horizontal placement of streams requires that operators use the best available aerial
photography, terrain data, and other background base data to achieve placement accuracy.
Aerial photography is a very good source of base data when performing the horizontal
placement of streams and water bodies; however, there are certain drawbacks when using this
data source. Smaller streams and water bodies are sometimes obscured by various natural and
manmade land use / ground cover features. For example, heavily-forested and urban areas
may hinder the operator’s ability to see stream or water body features.
In order to provide background information for determining the accuracy of horizontal placement
of streamlines and water bodies, ground cover characteristics and other pertinent information
may need to be determined and recorded in the North Carolina Streambed Mapping
Geodatabase for future reference for assessing confidence levels in the resulting data. Ground
cover characteristics refer to the degree of obscurity in areas where horizontal placement of
streamlines and water bodies is occurring for this project. It is hypothesized that the more
obscured an area, the more likely error will be introduced into the horizontal placement by the
operator. This may create a situation where the operator plays a major role in the accuracy
assessment process. It is not practical or feasible for the operator to populate ground
classifications while performing the horizontal placement of streamline and water body vectors.
Therefore, an automated methodology may be needed for generating the horizontal accuracy
estimates of streamlines and water bodies based upon ground classification in order to meet
production rate goals during horizontal placement, and to establishment of confidence levels
after horizontal placement. Moreover, an alternative solution to the recording of ground cover
characteristics may be needed.
Issues
The main issue is to determine an automated methodology for generating the horizontal
accuracy estimates of stream and water body vectors in order to meet production rate goals and
develop accuracy confidence levels for users of this data. This includes a methodology to
accurately determine, quantify and record the degree to which streams and water bodies are
visually obscured to the operator on the aerial photography during horizontal placement due to
vegetation, manmade structures, imperfections in the aerial photography, and other visual
impediments. These natural and manmade impediments hinder the operator by making it
difficult to see the physical boundaries of the stream or water body. Therefore, he or she is
forced to use their best judgment and/or use other technological aids for digitizing obstructed
features. These situations may cause the end-user of the data to have less confidence in the
accuracy of the horizontal placement of the stream or water body, compared to when the
operator can clearly see the feature on the aerial photography. The establishment of an
agreed-upon methodology for determining ground cover classifications and applying them to the
dataset may be necessary.
Ground cover Characteristic Values
Draft Date: 7/12/2006 (Rev5)
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NC Streambed Mapping Project
Issue Paper
In order to automate the horizontal accuracy estimates of stream and water body vectors, it may
be necessary to generate the ground characteristic categories to be used for this process.
There are numerous ground cover classifications available from various sources. However, the
focus of this paper is to determine the best source of these categories based on the results of
this project’s Accuracy Study.
The determination of ground cover characteristic categories and the automated application of
these categories will ensure that future users of this data are aware of the inherent data errors
created by ground cover obscurity. Furthermore, the determination of ground cover
characteristics is just one variable that will determine accuracy estimates. There are other
limitations the user must be aware of based upon the limitations of other base data used during
horizontal placement, including the horizontal accuracy of base data.
Methods
Method 1
One method of quantifying and recording the degree to which streams and water bodies are
visible to the operator is to use some type of ground cover coverage as the basis for
determining the percentage of the horizontally-placed features impacted by a particular land use
classification. A land use layer can be created by using the land use grids supplied from the
North Carolina Floodplain Mapping Program (NCFMP), which was created from the original
LIDAR data and intensity photography. The process to create a land use layer from the grid is
to convert the land use values in the value attribute table from float to integer, and then create a
polygon layer based on these integer values. This land use layer can be compared to the
stream and water body layers to auto-populate the necessary metadata tables in the North
Carolina Streambed Mapping Geodatabase for later reference.
Method 2
Another possible method for creating a ground cover classification layer is to have the operator
generate a polygonal layer attributed with ground cover categories shortly after the horizontal
placement of features. This polygonal layer could be digitized by the operator to reflect the
current, interpreted ground cover / obscurity based on the available base data. This layer could
subsequently be used in an automated process to transfer the ground classifications to the
stream and water body vectors, thus populating the proper fields in the “submetadata” table in
the geodatabase.
Ground classification categories can be determined based upon results of the Accuracy Study
because survey points were selected in multiple areas with varying degrees of obscurity based
on ground cover. These areas ranged from generally open, non-obscured areas to mostly
forested, obscured areas. The results of the Accuracy Study will clearly define the logical
ground classification categories by providing insight into the accuracy of horizontal placement in
each of these areas. This may include the combining of two logical categories into one category
that represents the true visibility of the stream and water body vectors using the available aerial
photography. For example, if the Accuracy Study determines that forested and urban areas
produce the same degree of obscurity, these classifications should be combined into one class.
Ground cover Characteristic Values
Draft Date: 7/12/2006 (Rev5)
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NC Streambed Mapping Project
Issue Paper
Currently, the “submetadata” table in the NC Streambed Mapping Project Geodatabase houses
fields to populate surface characteristics (ground cover), source orthophotography, LIDAR postspacing, terrain source data, orthophotography source date, and horizontal accuracy. The
determination of horizontal accuracy of a stream or water body feature can be achieved by
using a combination of these attributes that when combined, create a logical range representing
the horizontal accuracy of the stream and water body vectors.
Method 3
An alternative method is to eliminate ground classifications from the database. The use of
these types of classifications has been determined to be highly subjective based upon the local
knowledge of the operator. Moreover, there are numerous sources of land use / land cover or
ground cover classification datasets generated by multiple methodologies. This project does
not have any deliverables associated with it concerning the production of a ground cover
dataset. It was initially hypothesized that ground cover or obscurity would be strongly correlated
with horizontal placement accuracy. However, the Streambed Mapping for Western North
Carolina Counties Horizontal Accuracy Study Report concluded that there was little or no
correlation between the ground cover classifications originally targeted in the study, which
included forest, brush, open and urban. The only correlation suggested in the study is that the
operators reported and the data alluded to the fact that urban areas are particularly challenging
due to development, stream redirection and placement of streams underground.
The elimination of capturing the ground cover classification in the geodatabase would include
the deletion of the “AlnGrdChar” field and “Range” fields from the “submetadata” table. Also, it
would include the modification of the “HrzntlAcc” field by adding an associated domain. The
domain would be “A”, which would represent a reference to the Streambed Mapping for Western
North Carolina Counties Horizontal Accuracy Study Report. This would allow the user of the
data to learn and understand the horizontal accuracy of the data they are using. This report
refers exclusively to the mountainous region contained in the 19 Counties in western North
Carolina.
Recommendation
Because the “Streambed Mapping for Western North Carolina Counties Horizontal Accuracy
Study Report” proved that there is little or no correlation between ground cover and horizontal
accuracy, this group recommends that Method 3 be implemented to resolve this issue.
Therefore, the “AlnGrdChar” and “Range” fields should be deleted for the “submetadata” table.
Similarly, this group recommends that the “Range” field be deleted from the “submetadata” field
because they will not accurately depict the true accuracy of the data, and will be ambiguous to
the user community. Additionally, this group recommends that the “HrzntlAcc” field be modified
to be a domain field populated with a “A”, which will represent “Western North Carolina Counties
Accuracy Study Report.” This will allow the user to learn more about the true accuracy of the
data contained in the 19-County region of western North Carolina as to the probability that there
data is within a certain horizontal distance from the true location of the streambed.
Discussion Summary
Date Discussed: Rev1 – March 9, 2006
Ground cover Characteristic Values
Draft Date: 7/12/2006 (Rev5)
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NC Streambed Mapping Project
Issue Paper
Discussion Participants: Joe Sewash, Andy McDaniel, Scott Edelman, Aaron Lowery, and
Roger Cottrell
Summary of Discussion
3/9/06 (Rev1) – Focus of this paper changed from the actual mechanics of populating the GDB
with ground classifications to the issue of a methodology of determining ground classification
based on the operator interpretation of imagery where streams and water bodies are obscured.
This interpretation will be aided by the results of the in-progress Accuracy Study because the
Accuracy Study determines the actual placement accuracy in areas that are obscured or not
obscured due to various ground characteristics. Moreover, the discussion addressed possible
automated solutions for using ground characteristics and other metadata in the determination of
horizontal accuracy reporting to end-users of the data.
Date Discussed: Rev2 – March 20, 2006
Discussion Participants: Andy McDaniel, Aaron Lowery and Roger Cottrell
Summary of Discussion
3/20/06 (Rev2) – A simpler solution to the population of the horizontal accuracy in the
geodatabase is an accuracy range populated in the geodatabase that reflects the best and
worst case accuracies for each stream line feature for the 19-county areas. For example, if the
worst case is 20 feet in forested areas and the best is 3 feet in open, the range would be 3 – 20
feet for the 19 county-area (Phase I). This would allow the user to get a general accuracy range
for the area without overstating the true accuracy of the horizontal placement of stream lines
and water bodies.
Andy McDaniel also suggested that there be an accuracy study for subsequent regions of NC
based on the general topography. For example, if the next phase of the project is to horizontally
place stream lines and water bodies in the Piedmont, another accuracy study should be
conducted to ascertain a range for that region to cover Phase II. Phase II may include another
region of NC – the coastal plain, which will require a third accuracy study.
Date Discussed: Rev3 – May, 4, 2006
Discussion Participants: Andy McDaniel, Joe Sewash, Roger Cottrell and Ben Pope
Summary of Discussion
5/4/06 (Rev3) - The group came to a quick consensus that this discussion needed to be
postponed until the finalization of the Accuracy Study Report. The entire ground cover /
accuracy issue is dependent upon the results of this study. Particular attention needs to be
given to creating tables and graphs in the accuracy study that focus on the probability that any
given stream segment is within a certain accuracy.
Date Discussed: Rev4 – July 11, 2006
Discussion Participants: Andy McDaniel, Joe Sewash, Aaron Lowery and Roger Cottrell
Summary of Discussion
Ground cover Characteristic Values
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NC Streambed Mapping Project
Issue Paper
Andy McDaniel suggested to the group that there be one, generic set of data for accuracy for
the entire 19-County area with a range of probabilities. Joe pointed out that a range of
probabilities may not be practical and may confuse the users. The group came to a consensus
that the “range” field should be eliminated from the “submetadata” table.
The discussion then focused on the “AlnGrdChar” field in the “submetadata” table. This field
was originally slated to hold attribution for “Bare”, “Mixed” and “Heavy” domain values that
represented the ground cover characteristics observed by the horizontal placement operators.
Because the accuracy study showed little to no correlation between ground cover and horizontal
accuracy, it was decided by the group that this field is not useful to the user either. Joe
suggested we remove this field from the “submetadata” table. This was in support of Andy’s
thoughts that the ground cover classifications were derived using too much subjectivity and
would not be able to be logically backed by supporting data.
The discussion concerning the “HrzntlAcc” field in the “submetadata” table focused on what this
field should be populated with to allow the user of the data to get a good understanding of the
horizontal accuracy of the streamlines. One suggestion was to populate this field with a “worstcase” distance value in either feet or meters. The group decided this was not a good idea
because it would be ambiguous to the user. The group came to a unanimous decision to
populate this field with a domain value of “A”, which would represent a text value for “Western
North Carolina Counties Accuracy Study Report”. The thought behind this decision is that
subsequent mappings of North Carolina will require separate accuracy studies that represent
the terrain characteristics of that region of North Carolina. For example, the Piedmont will
require a different set of accuracy study findings based on its differences with the mountainous
region of North Carolina. This will require that the user read the accuracy study to learn more
about the accuracy of the data they are using for their particular application.
Final Guidelines
Method 3 will be implemented to finalize the issues contained in this paper. This will allow the
user to learn more about the true accuracy of the data contained in the 19-County region of
western North Carolina as to the probability that there data is within a certain horizontal distance
from the true location of the streambed. The “AlnGrdChar” and “Range” fields will be removed
from the geodatabase schema and the “HrzntlAcc” field will be modified to accommodate a
domain with the value of “A” representing the Streambed Mapping for Western North Carolina
Counties Horizontal Accuracy Study Report.
Ground cover Characteristic Values
Draft Date: 7/12/2006 (Rev5)
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