WEBVTT Kind: captions Language: en 00:00:07.180 --> 00:00:17.480 3DEP Topic Lesson: Digital Elevation Models, Hydro-Flattening, Hydro-Enforcement, and Breaklines 00:00:17.480 --> 00:00:24.200 In this 3DEP Topic lesson, we will provide information on different types of Digital 00:00:24.200 --> 00:00:31.760 Elevation Models, or DEMs. We will discuss the difference between a Topographic DEM and 00:00:31.760 --> 00:00:38.610 a Hydrologic DEM, and provide explanations of the related terms “hydro-flattened” 00:00:38.610 --> 00:00:46.770 and “hydro-enforced”. We will also provide a basic explanation of breaklines and the 00:00:46.770 --> 00:00:56.030 purposes they serve. Understanding the differences between these two types of DEMs, the concepts 00:00:56.030 --> 00:01:03.010 of hydro-flattening and hydro-enforcement, and the purposes that breaklines serve, are 00:01:03.010 --> 00:01:09.710 important steps in better utilizing 3DEP products. 00:01:09.710 --> 00:01:16.049 By the end of this lesson, you will be able to determine which type of DEM is best suited 00:01:16.049 --> 00:01:23.909 for specific purposes, understand the difference between the concepts of hydro-flattening and 00:01:23.909 --> 00:01:29.450 hydro-enforcement, and identify the purpose and value of breaklines. 00:01:29.450 --> 00:01:38.189 In general, there are two types of Digital Elevation Models: Topographic DEMs, which 00:01:38.189 --> 00:01:46.440 are used for mapping purposes, and Hydrologic DEMs, which are used for modeling purposes. 00:01:46.440 --> 00:01:52.090 We will describe each of the elevation models listed here in more detail, starting with 00:01:52.090 --> 00:01:56.550 four types of topographic DEMs. 00:01:56.550 --> 00:02:06.330 Traditionally, Digital Terrain Models (DTMs) are the output from stereo compilation of 00:02:06.330 --> 00:02:15.809 vector masspoints and breaklines from digital orthophotos. DTMs are of a coarser resolution 00:02:15.809 --> 00:02:24.400 than DEMs from lidar. They visually depict the characteristics of a topographic DEM including: 00:02:24.400 --> 00:02:32.349 flat water surfaces, removal of bridges, defined road edges, and retention of road fills over 00:02:32.349 --> 00:02:40.069 drainage culverts. A pure lidar DEM is a representation of the 00:02:40.069 --> 00:02:48.269 surface, created strictly from the lidar point cloud data. This type of DEM is problematic 00:02:48.269 --> 00:02:55.579 for mapping purposes due to the unacceptable appearance of water surfaces. Water surfaces 00:02:55.579 --> 00:03:02.480 do not provide lidar return, which results in the triangulation artifacts, or “tinning” 00:03:02.480 --> 00:03:09.480 that you see in this image. The pure lidar DEM has no breaklines to constrain the water 00:03:09.480 --> 00:03:16.059 surface and define the banks. The appearance of the water surface renders this type of 00:03:16.059 --> 00:03:26.120 DEM as unacceptable, cartographically speaking. The simple Hydro-Flattened DEM is a representation 00:03:26.120 --> 00:03:32.279 of the surface that is cartographically acceptable in appearance, and is suitable for creating 00:03:32.279 --> 00:03:39.109 contours. Offensive triangulation artifacts have been removed. Note the different appearance 00:03:39.109 --> 00:03:46.109 of the water features compared to the previous slide. The hydro-flattened DEM as a deliverable 00:03:46.109 --> 00:03:53.620 is a goal of the USGS Lidar Base Specification. Please note that hydro-flattening is purely 00:03:53.620 --> 00:04:00.010 a cartographic enhancement. Although the waterbodies are considered to have a single elevation, 00:04:00.010 --> 00:04:06.579 that elevation is estimated from the adjacent terrain and is not representative of any measured 00:04:06.579 --> 00:04:12.989 water surface elevation, and should not be used for calculation of water volume or flow, 00:04:12.989 --> 00:04:22.040 or any other hydrologic or hydraulic modeling.  Topographic contours created from hydro-flattened DEMs 00:04:22.040 --> 00:04:27.110 are created with the bridge decks removed from the model, but road surfaces are left 00:04:27.110 --> 00:04:35.860 intact over drainage culverts. The enhanced Hydro-flattened DEM has all the 00:04:35.860 --> 00:04:42.680 characteristics of the simple hydro-flattened DEM, but with further delineation of features 00:04:42.680 --> 00:04:52.730 such as roads, single line drainages, ridges, bridge crossings, and buildings. Such delineation 00:04:52.730 --> 00:04:59.379 requires a large number of additional breaklines, which substantially increases the amount 00:04:59.379 --> 00:05:07.590 of time and effort expended, and increases the costs needed to create this surface representation. 00:05:07.590 --> 00:05:12.860 Although the result is a very high quality surface model, it is not cost effective for 00:05:12.860 --> 00:05:20.491 the 3DEP program. So far, we have discussed four types of Topographic 00:05:20.491 --> 00:05:29.889 DEMs. Now let’s look at two types of Hydrologic DEMs. The Hydro-Enforced DEM is a surface 00:05:29.889 --> 00:05:36.990 used by engineers for Hydraulic and Hydrologic (H&H) modeling. Although similar to the Hydro-Flattened 00:05:36.990 --> 00:05:43.750 DEM, it has additional surface modifications to “allow” water to flow across the surface, 00:05:43.750 --> 00:05:50.390 as it does in the real world. The most notable modification is that road fills are “cut 00:05:50.390 --> 00:05:56.650 through” at drainage culverts. Notice the “cuts” in the road surfaces in the image, 00:05:56.650 --> 00:06:02.970 allowing water to “flow” across the surface. This representation of hydro-enforcement is 00:06:02.970 --> 00:06:09.319 critical for accurate modeling, but not useful for traditional mapping purposes or creation 00:06:09.319 --> 00:06:17.810 of topographic contours. Another type of hydrologic DEM is the Hydro-Conditioned 00:06:17.810 --> 00:06:26.419 DEM. This DEM is similar to the Hydro-Enforced surface, but with additional treatments. 00:06:26.419 --> 00:06:32.430 In most landscapes there are depressions that have no internal drainage path. These are 00:06:32.430 --> 00:06:38.080 known as “sinks”. When it rains, these depressions fill until the water reaches a 00:06:38.080 --> 00:06:44.620 downhill outlet path, where the water is allowed to escape the sink. The location where this 00:06:44.620 --> 00:06:51.680 occurs is known as a “pour point”. In a topographic DEM, these depressions are perfectly 00:06:51.680 --> 00:06:59.050 normal and are retained in the surface model. However, in some hydrologic modeling applications, 00:06:59.050 --> 00:07:06.360 they can cause erratic behavior. To avoid this, a hydro-enforced DEM is “filled”, 00:07:06.360 --> 00:07:11.909 raising the elevation of the ground surface within each sink until the elevation of its 00:07:11.909 --> 00:07:18.870 pour point is reached. The result is a flattened ground area over which modeled water can flow 00:07:18.870 --> 00:07:25.870 unimpeded across the entire surface, with no areas of unconnected internal drainage. 00:07:25.870 --> 00:07:31.889 This type of hydrologic DEM is known as a Hydro-Conditioned DEM. 00:07:31.889 --> 00:07:40.050 Next, let’s discuss breaklines. Breaklines are linear or polygonal features that describe 00:07:40.050 --> 00:07:48.389 a change in the smoothness or continuity of a surface. Breaklines ensure that known elevation 00:07:48.389 --> 00:07:54.870 values along a linear feature, or within a polygonal feature, are maintained with precision 00:07:54.870 --> 00:08:04.720 and continuity. This goal cannot be achieved using masspoints (namely, lidar points) alone. 00:08:04.720 --> 00:08:11.610 Breaklines are used to represent features such as drainage ditches, streams and rivers, 00:08:11.610 --> 00:08:20.830 ridges, road crowns and curbs, bridges, building footprints ... most anything that is or creates 00:08:20.830 --> 00:08:29.599 a linear change in elevation or slope. Breaklines are usually maintained as 3D features, where 00:08:29.599 --> 00:08:38.510 each vertex has its own x, y, and Z coordinate values. Some breaklines, such as the bank 00:08:38.510 --> 00:08:45.920 of a lake or pond, can be represented in 2D with a single elevation attribute, as all 00:08:45.920 --> 00:08:52.060 the vertices should have the same Z-value, but it is generally preferred to maintain 00:08:52.060 --> 00:08:58.810 all breaklines in 3D. Breaklines are used to create hydro-flattened 00:08:58.810 --> 00:09:05.990 DEMs by defining the edges of waterbodies, so that the water can then be “flattened”, 00:09:05.990 --> 00:09:14.190 that is, the entire water surface is constrained. Smaller closed lakes and ponds will be uniformly 00:09:14.190 --> 00:09:22.160 flat and level; wide rivers and reservoirs will exhibit a downhill gradient (also referred 00:09:22.160 --> 00:09:29.130 to as monotonic flow) as they do in nature, while remaining level from bank-to-bank. For 00:09:29.130 --> 00:09:37.130 the purposes of hydro-flattening, most breaklines will be polygonal, though single-line centerlines 00:09:37.130 --> 00:09:43.800 are needed in gradient water bodies to expedite production. 00:09:43.800 --> 00:09:50.810 Additional hydrologic breaklines can further be used to represent narrow streams, drainages, 00:09:50.810 --> 00:09:58.120 and other hydrologic features. Single-line stream breaklines can enhance a DEM surface 00:09:58.120 --> 00:10:05.720 by introducing distinct, lowered “creases” into the surface, ensuring that the drainage 00:10:05.720 --> 00:10:13.160 is the lowest and uninterrupted path. Breaklines for culverts support the development of hydro-enforced 00:10:13.160 --> 00:10:20.810 DEMs, used in modeling applications. Note that hydrologic breaklines may never rise 00:10:20.810 --> 00:10:25.079 above the surrounding land surface. 00:10:25.079 --> 00:10:34.399 To review, here again are the images depicting each type of digital elevation model: 00:10:34.399 --> 00:10:40.989 • Stereo Topographic Digital Terrain Model (DTM) 00:10:40.989 --> 00:10:45.839 • Pure Lidar Topographic DEM: Lidar points only 00:10:45.839 --> 00:10:53.940 • Hydro-Flattened Topographic (Simple) DEM • Hydro-Flattened Topographic (Enhanced) 00:10:53.940 --> 00:10:57.710 DEM • Hydro-Enforced Topographic DEM 00:10:57.710 --> 00:11:08.589 • Hydro-Conditioned Topographic DEM In this 3DEP topic lesson, we demonstrated 00:11:08.589 --> 00:11:17.480 how to determine which type of DEM is best suited for specific purposes, provided information 00:11:17.480 --> 00:11:25.580 on the differences between the concepts of hydro-flattening and hydro-enforcement, and 00:11:25.580 --> 00:11:33.050 identified the purpose and value of breaklines. Understanding these concepts is of importance 00:11:33.050 --> 00:11:39.949 in order to better utilize 3DEP products.