3DEP Topic Lesson: Digital Elevation Models, Hydro-Flattening, Hydro-Enforcement, and Breaklines
In this 3DEP Topic lesson, we will provide information on different types of Digital
Elevation Models, or DEMs. We will discuss the difference between a Topographic DEM and
a Hydrologic DEM, and provide explanations of the related terms “hydro-flattened”
and “hydro-enforced”. We will also provide a basic explanation of breaklines and the
purposes they serve. Understanding the differences between these two types of DEMs, the concepts
of hydro-flattening and hydro-enforcement, and the purposes that breaklines serve, are
important steps in better utilizing 3DEP products.
By the end of this lesson, you will be able to determine which type of DEM is best suited
for specific purposes, understand the difference between the concepts of hydro-flattening and
hydro-enforcement, and identify the purpose and value of breaklines.
In general, there are two types of Digital Elevation Models: Topographic DEMs, which
are used for mapping purposes, and Hydrologic DEMs, which are used for modeling purposes.
We will describe each of the elevation models listed here in more detail, starting with
four types of topographic DEMs.
Traditionally, Digital Terrain Models (DTMs) are the output from stereo compilation of
vector masspoints and breaklines from digital orthophotos. DTMs are of a coarser resolution
than DEMs from lidar. They visually depict the characteristics of a topographic DEM including:
flat water surfaces, removal of bridges, defined road edges, and retention of road fills over
drainage culverts. A pure lidar DEM is a representation of the
surface, created strictly from the lidar point cloud data. This type of DEM is problematic
for mapping purposes due to the unacceptable appearance of water surfaces. Water surfaces
do not provide lidar return, which results in the triangulation artifacts, or “tinning”
that you see in this image. The pure lidar DEM has no breaklines to constrain the water
surface and define the banks. The appearance of the water surface renders this type of
DEM as unacceptable, cartographically speaking. The simple Hydro-Flattened DEM is a representation
of the surface that is cartographically acceptable in appearance, and is suitable for creating
contours. Offensive triangulation artifacts have been removed. Note the different appearance
of the water features compared to the previous slide. The hydro-flattened DEM as a deliverable
is a goal of the USGS Lidar Base Specification. Please note that hydro-flattening is purely
a cartographic enhancement. Although the waterbodies are considered to have a single elevation,
that elevation is estimated from the adjacent terrain and is not representative of any measured
water surface elevation, and should not be used for calculation of water volume or flow,
or any other hydrologic or hydraulic modeling. Topographic contours created from hydro-flattened DEMs
are created with the bridge decks removed from the model, but road surfaces are left
intact over drainage culverts. The enhanced Hydro-flattened DEM has all the
characteristics of the simple hydro-flattened DEM, but with further delineation of features
such as roads, single line drainages, ridges, bridge crossings, and buildings. Such delineation
requires a large number of additional breaklines, which substantially increases the amount
of time and effort expended, and increases the costs needed to create this surface representation.
Although the result is a very high quality surface model, it is not cost effective for
the 3DEP program. So far, we have discussed four types of Topographic
DEMs. Now let’s look at two types of Hydrologic DEMs. The Hydro-Enforced DEM is a surface
used by engineers for Hydraulic and Hydrologic (H&H) modeling. Although similar to the Hydro-Flattened
DEM, it has additional surface modifications to “allow” water to flow across the surface,
as it does in the real world. The most notable modification is that road fills are “cut
through” at drainage culverts. Notice the “cuts” in the road surfaces in the image,
allowing water to “flow” across the surface. This representation of hydro-enforcement is
critical for accurate modeling, but not useful for traditional mapping purposes or creation
of topographic contours. Another type of hydrologic DEM is the Hydro-Conditioned
DEM. This DEM is similar to the Hydro-Enforced surface, but with additional treatments.
In most landscapes there are depressions that have no internal drainage path. These are
known as “sinks”. When it rains, these depressions fill until the water reaches a
downhill outlet path, where the water is allowed to escape the sink. The location where this
occurs is known as a “pour point”. In a topographic DEM, these depressions are perfectly
normal and are retained in the surface model. However, in some hydrologic modeling applications,
they can cause erratic behavior. To avoid this, a hydro-enforced DEM is “filled”,
raising the elevation of the ground surface within each sink until the elevation of its
pour point is reached. The result is a flattened ground area over which modeled water can flow
unimpeded across the entire surface, with no areas of unconnected internal drainage.
This type of hydrologic DEM is known as a Hydro-Conditioned DEM.
Next, let’s discuss breaklines. Breaklines are linear or polygonal features that describe
a change in the smoothness or continuity of a surface. Breaklines ensure that known elevation
values along a linear feature, or within a polygonal feature, are maintained with precision
and continuity. This goal cannot be achieved using masspoints (namely, lidar points) alone.
Breaklines are used to represent features such as drainage ditches, streams and rivers,
ridges, road crowns and curbs, bridges, building footprints ... most anything that is or creates
a linear change in elevation or slope. Breaklines are usually maintained as 3D features, where
each vertex has its own x, y, and Z coordinate values. Some breaklines, such as the bank
of a lake or pond, can be represented in 2D with a single elevation attribute, as all
the vertices should have the same Z-value, but it is generally preferred to maintain
all breaklines in 3D. Breaklines are used to create hydro-flattened
DEMs by defining the edges of waterbodies, so that the water can then be “flattened”,
that is, the entire water surface is constrained. Smaller closed lakes and ponds will be uniformly
flat and level; wide rivers and reservoirs will exhibit a downhill gradient (also referred
to as monotonic flow) as they do in nature, while remaining level from bank-to-bank. For
the purposes of hydro-flattening, most breaklines will be polygonal, though single-line centerlines
are needed in gradient water bodies to expedite production.
Additional hydrologic breaklines can further be used to represent narrow streams, drainages,
and other hydrologic features. Single-line stream breaklines can enhance a DEM surface
by introducing distinct, lowered “creases” into the surface, ensuring that the drainage
is the lowest and uninterrupted path. Breaklines for culverts support the development of hydro-enforced
DEMs, used in modeling applications. Note that hydrologic breaklines may never rise
above the surrounding land surface.
To review, here again are the images depicting each type of digital elevation model:
• Stereo Topographic Digital Terrain Model (DTM)
• Pure Lidar Topographic DEM: Lidar points only
• Hydro-Flattened Topographic (Simple) DEM • Hydro-Flattened Topographic (Enhanced)
DEM • Hydro-Enforced Topographic DEM
• Hydro-Conditioned Topographic DEM In this 3DEP topic lesson, we demonstrated
how to determine which type of DEM is best suited for specific purposes, provided information
on the differences between the concepts of hydro-flattening and hydro-enforcement, and
identified the purpose and value of breaklines. Understanding these concepts is of importance
in order to better utilize 3DEP products.