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Designing A Mapping Project Brian MayfieldProposal Coordinator / GIS Mapping
Scientist
Surdex Corporation520 Spirit of St. Louis
Blvd. St. Louis, MO 63005 Phone: 636-532-3427 Fax: 636-537-9638
Abstract: Recent market analyses indicate that the number of counties and municipalities that have implemented an enterprise GIS for their community has grown exponentially in the last five years. Increasingly, these communities are turning to the photogrammetric industry to provide accurate and precise mapping data for use in their enterprise system. This paper examines some of the considerations a community must give when developing a mapping project. Introduction Surdex Corporation has served County, Municipal,
State, Federal and Private Agencies since
1954. During that period, we have been recognized as a premier geospatial
data provider; supplying accurate and precise information to our customers on
time and within budget. We attribute our longevity and success to an
unparalleled understanding of our clients needs and goals. Unlike many of our
competitors, our objective is to help you meet your goals accurately, on
time, and within budget. The following sections should provide a starting point for the development of a county or municipal mapping project. Essential elements of a mapping project such as selecting a final product accuracy, photo scale, and contour interval are discussed hereafter. Product Accuracy The first step in developing a mapping project is to identify all current projects and to forecast any potential projects that might use the data produced for a mapping project by a photogrammetric vendor. The uses of a mapping product often dictate the type of data that will be needed and the accuracy of the data that must be obtained. Accuracy standards vary in complexity and
usability. The most commonly used data
accuracy standards for county and municipal are the American Society of Photogrammetry
and Remote Sensing (ASPRS) Class I and II.
Additionally, more and more counties and municipalities are requesting
their mapping projects to be compliant with the National Map Accuracy Standards
(NMAS) for large-scale mapping. NMAS
generally equates to ASPRS Class 1.5. The following tabular information is excerpted from the USACE Engineering and Design Manual for Photogrammetric
Production. It illustrates the
Limiting horizontal Root Mean Square Error (RMSE) for each class of accuracy as
it relates to mapping scale.
The map scales listed above are indicative of the
most popular map scales requested by counties and municipalities for their
mapping projects. The RMSE is the
square root of the quotient of the sum of the squares of the errors divided by
the number of measurements. In other
words, the 1=100 scale mapping compliant with National Map Accuracy Standards
would have a horizontal accuracy greater than or equal to +/- 1.5 feet. The product accuracy also significantly increases
project costs and schedules. Many counties and municipalities often fall short
of meeting their goals by assigning the wrong product accuracy specifications
to their project. The product accuracy should be a derivative of your
communities mapping needs and budget constraints. They falter by assigning a very strict product accuracy (e.g.
ASPRS Class I), thus limiting the amount of mapping data they can receive
within their budget. Choosing the Photo Scale Choosing the correct photo scale, which
is the flying height (Above Ground Level) divided by the cameras focal length
is the key to a successful mapping project. Surdex has performed thousands of
aerial photography missions during our 45 plus years of experience. This has provided us with an intimate
knowledge of the limitations of photographic and photogrammetric
equipment. Surdex works regularly with the US Army
Corps of Engineers to test and recommend mapping specifications. The following tables demonstrate the maximum
allowable photo scales for the features mentioned in the Alachua County Request
for Information. The first table illustrates the minimum
negative scale (or photo scale) needed for topographic development. The table,
which is excerpted from the USACE Engineering and Design Manual for
Photogrammetric Production, shows the Contour Interval in Feet and the values
that would need to be used to achieve a product accuracy of ASPRS Class I and
II as well as NMAS.
As you can see, the contour interval
will greatly affect the photo scale.
Also, the lower photo scale will result in an increased cost for aerial
photography acquisition and will greatly increase the minimum amount of models
that will need to bet set up for photogrammetric compilation of the Digital
Terrain Model and the requested planimetric features. When combined, these factors will increase the project
schedule. When constrained by budget, many
counties or municipalities decide to acquire digital orthophotography and/or
planimetric features only. In this
case, we refer to another table from the USACE Engineering and Design Manual
for Photogrammetric Production. This
table illustrates the minimum negative scales (photo scale) needed for Digital
Elevation Model Extraction and Planimetric Mapping.
As you can see, the overall project cost
would be reduced significantly by eliminating the contour component from the
mapping. The most important item when selecting mapping features to be produced
for a project is to take careful examination of usefulness and necessity of
certain types of data such as 1 contours.
More information about the selection of the appropriate contour interval
for various applications will be further discussed in this document. Control and Aerial Triangulation
Fully Analytical Aerial Triangulation For over three decades, Surdex has been perfecting the science of Fully Analytical Aerial Triangulation. All of our FAAT is performed in-house using only First-Order Fully Analytical SoftPlotter Instruments and experienced technicians. Surdex has long been a recognized leader in the science of FAAT, providing this service to other photogrammetric firms as well as to government entities and private agencies. The
purpose of aerial triangulation in the photogrammetric production process is to
establish precise and accurate relationships between the individual
photographic film coordinate systems and a defined datum and projection. This
relationship is used to link the ground surveyed control points via
photographic measurements. The result of the triangulation is a densified set
of ground control points that are used to control the remainder of the mapping
process. The maximum allowable error for the triangulation
process is demonstrated in the following table, which was excerpted from the
USACE Engineering and Design Manual for Photogrammetric Production.
H represents flying height (above ground level) in the table above. These computations provide us with the maximum allowable error for the aerial triangulation process, making it easy to identify errors or deviations from the desired product accuracy. DTM or DEM Collection A DTM (Digital Terrain Model), which is needed for
the generation of contours, is a highly accurate representation of ground
surface using mass points and breaklines.
A DEM (Digital Elevation Model), which required for the generation of
digital orthophotography, is a less accurate representation of the ground
surface using a regularly spaced grid of mass points and breaklines. Surdex
regularly compiles both types of surfaces for county and municipal
projects. However, Surdex will not
compile a DEM in addition to a DTM if the project deliverables include
contours. We will simply use the DTM in
the development of the digital orthophotography. The required mapping product (scale and accuracy)
determines the placement density of mass points regardless of whether the
surface is a DEM or DTM. Mass points consist of individual points on the
surface of the earth whose three space coordinates are measured by the
operator. The compilation process for a DTM or DEM involves the collection of a dense pattern of mass points and breaklines in the stereo model. Breaklines are collected along points of inflection in the topographic surface of the earth, i.e., places where there are sharp changes in the direction of slope on the earth's surface. Some examples where breaklines are placed are at the edge of road surfaces, bottom of creek beds, along hydrographic features and along the top of ridgelines. Breaklines consist of a string of mass points that are uniquely tied together by a computer code. Contours The tables shown in the previous sections illustrate how the selection of the appropriate contour interval for a countywide project will greatly influence the overall project cost and schedule. The following table is excerpted from the USACE Engineering and Design Manual for Photogrammetric Production. It provides the recommended use for contours and we recommend that Alachua County closely evaluate these recommendations. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
