Last updated 01/10/97
Ferdi Hellweger

AGREE - DEM Surface Reconditioning System

TABLE OF CONTENTS

INTRODUCTION

AGREE is a surface reconditioning system for Digital Elevation Models (DEMs). The system adjusts the surface elevation of the DEM to be consistent with a vector coverage. The vector coverage can be a stream or ridge line coverage. The system is written in ARC/INFO's Arc Macro Language (AML).

A quick overview of the procedure is as follows:

  1. Drop/raise the elevation of the cells corresponding to the vector lines a certain amount (smoothdist).
  2. Buffer the vector lines (buffer).
  3. Assign elevation to the cells inside the buffer so that there is a straight line path from the vector line to the original elevation just outside the buffer.
  4. Drop/raise the elevation of the cells corresponding to the vector lines a certain amount (sharpdist). This was added in Version 1.1.
Figures 2 and 3 are shaded pictures of an original and a surface reconditioned DEM, respectively.

Figure 2. Original DEM.

Figure 3. Surface Reconditioned DEM.
This system is an alternative to the 'burning in the streams' process which simply drops the elevation of the cells corresponding to the streams (followed by a filling of the newly created sinks).

The next section describes the methodology of the system (including on-line visualization with Java). Then a few points for usign the system are discussed. Finally a case study is presented comparing surface reconditioning to 'burning in the streams'.

WHAT'S NEW IN VERSION 1.1

AGREE Version 1.1 has the added capability to drop/raise the cells of the DEM corresponding to the vector lines a certain amount. This is effectively a 'burning in the streams' after the original AGREE proceedure.

The reason for making the revision is as follows. Most of the sinks in the DEM are located on the streams. This means that filling after the original AGREE proceedure could wipe out the smoothed channel upstream of the sink. This can be eliminated by droping the stream a large distance. That way any filling done to remove sinks in the stream remains in the trench of the stream.

METHODOLOGY

A quick overview of the proceedure was presented in the introduction. Figure 1 illustrates the concept interactively (Java capable browser is needed). The figure is a cross section on a stream. The surface elevation and stream location can be changed by 'dragging' and the smooth drop/raise, sharp drop/raise and buffer distance can be specified as well.

Figure 1. Surface Reconditioning Methodology( Java source code ).

Proceedure:

  1. Compute the vector grid (vectgrid). The cells in the vector grid corresponding to the lines in the vector coverage have data. All other cells have no data.
  2. Compute the smooth drop/raise grid (smogrid). The cells in the smooth drop/raise grid corresponding to the vector lines have an elevation equal to that of the original DEM (oelevgrid) plus a certain distance (smoothdist). All other cells have no data.
  3. Compute the vector distance grids (vectdist and vectallo). The cells in the vector distance grid (vectdist) store the distance to the closest vector cell. The cells in vector allocation grid (vectallo) store the elevation of the closest vector cell.
  4. Compute the buffer grid (bufgrid2). The cells in the buffer grid outside the buffer distance (buffer) store the original elevation. The cells in the buffer grid inside the buffer distance have no data.
  5. Compute the buffer distance grids (bufdist and bufallo). The cells in the buffer distance grid (bufdist) store the distance to the closest valued buffer grid cell (bufgrid2). The cells in buffer allocation grid (bufallo) store the elevation of the closest valued buffer cell.
  6. Compute the smooth modified elevation grid (smoelev). The cells in the smooth modified elevation grid store the results of the smooth surface reconditioning process. Note that for cells outside the buffer the the equation below assigns the original elevation.
  7. Compute the sharp drop/raise grid (shagrid). The cells in the sharp drop/raise grid corresponding to the vector lines have an elevation equal to that of the smooth modified elevation grid (smoelev) plus a certain distance (sharpdist). All other cells have no data.
  8. Compute the modified elevation grid (elevgrid). The cells in the modified elevation grid store the results of the surface reconditioning process. Note that for cells outside the buffer the the equation below assigns the original elevation.

USING THE SYSTEM

Getting the System

The system consists of one AML file called agree.aml. The file can be downloaded from our anonymous ftp site:

The file can also be downloaded from this site: Starting the System

The system can be started from GRID with the AML &RUN directive as follows:

Alternatively the input parameters can be passed to the system as follows: Input Data Description
  1. Elevation Grid. The original elevation grid (DEM) of the area. The DEM does not have to be 'filled' in advance.
  2. Vector Coverage. The stream or ridgeline coverage of the area.
  3. Buffer Distance. The buffer distance controls the spatial extent of the surface reconditioning. This distance should be set roughly equal to or slightly larger than the approximate spatial scale of error among the elevation grid and vector coverage. The spatial scale of the error can be determined by delineating streams or subbasins from the original DEM and comparing those streams to the lines in the vector coverage. If, for example, the two lines are found to vary at most by about 400 meters, the buffer distance might be set to 600 meters.
  4. Smooth Drop/Raise Distance. The smooth drop/raise distance controls how much the cells corresponding to the vectors are droped/raised. In order to develop some guidelines for the smooth drop/raise distance a formula was developed which normalizes the distance based on the average surface slope inside the buffer and the buffer distance:
    5. The forcing factor controls the magnitude of the alteration. If it is set to 0.00 the vector cells will remain at the original elevation and the slope inside the buffer will be fairly close to the original one. A factor of 0.50 will result in a rough doubling of the slope inside the buffer. A factor of 0.10 was found to work well for the Tenkiller Reservoir drainage area (see the case study below). It is important to realize that this calculation should be used only to get a rough idea of the order of magnitude for the smooth drop/raise distance.
  5. Sharp Drop/Raise Distance. The sharp drop/raise distance controls how much the cells corresponding to the vectors are droped/raised after the smooth modified elevation grid is computed. This is essentially digging a trench/raising a wall. If vector elevation for the subsequent hydrologic analysis is not essential the sharp drop/raise distance should be large (i.e. 1000).
Post-Processing the Data

The modified elevation grid is saved as 'elevgrid'. Note that the system does not fill sinks. Before doing hydrologic calculations with the modified elevation grid sinks should be filled with the grid FILL command.

CASE STUDY: SURFACE RECONDITIONING VS. BURNING IN STREAMS

The Tenkiller Reservoir drainage basin was used as a case study. The filled DEM was used as input elevation grid. The RF1 stream coverage was used as input vector grid. Modified elevation grids were created using the 'burning in the streams' method and the surface reconditioning method. Streams were then delineated from both the two different modified elevation grids using the grid FLOWDIRECTION, FLOWACCUMULATION, CON and STREAMLINE functions.

A part of the study area which illustrates the differences among the two methods was chosen for the subsequent discussion. The paragraphs and figures below outline the major differences in the two methods.

TXDOT Example Slides

AGREE was used on the TXDOT study region. The 500 m DEM and Rf1 stream coverage were used as input. The DEM and the RF1 agree in areas of pronounced topography and disagree in flat areas (especially near the coast). In areas of pronounced topography the spacial scale of error is less than 500 m. The streams delineated from the AGREE DEM are therefore similar to those delineated from the burned DEM. In flat areas the spacial scale of error is significantly larger. The topography is so flat that it does not matter if streams are burned in or AGREE is used. The water will take the closest path to the Rf1 in either case. The streams delineated from the AGREE DEM are therefore, again, similar to those delineated from the burned DEM.

Below are some slides illustrating an example of where the stream network delineated from the AGREE DEM is significantly different from that delineated from the burned DEM. The whole region was checked at this scale. There are only a few cases where the difference is this significant.

Input Parameters:

Slides:
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