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Table of Contents
Table of Contents
- Background
- Description and Results
- Task 1: Build DEM from GPS Data of Pats Cabin Reach
- Task 2: Build DEM of Valley around Pats Cabin Reach from Lidar
- Task 3: Analyze DEMs to detect change in channel over time
- Description and Results
- Summary
- Reference
Background
Background
This lab aimed at learning how to build DEMs from raw topographic survey data and processed airborne LiDaR data. It also involved understanding how to manipulate raster datasets and doing change detection calculation on multi-years DEMs.
I have been asked to build some DEMs from raw data provided by a client for a new project in Oregon and to create various figures to illustrate the differences among the various DEM data available for this project.
Find below the location of interest with the TINs created from the topographic points provided. Click here or image for larger view.
Description and Results
Description and Results
Task 1: Build DEM from GPS Data of Pats Cabin Reach
Various DEMs (5cm, 10cm, 25cm, 50cm, ,1m ,2m, 5m and 10m) were built from the final TIN made using the survey points and breaklines provided. This were compared in the figure on the left below. The water depth was overlayed in the recommended resolution (25cm DEM). Click here for detailed instruction.
Click images for larger view.
The DEMs with smaller resolution seems clearer namely 5cm, 10cm, and 25cm. The bigger resolution are pixelated, and this increases with increase in resolution such that the ground feature becomes blurry. I have chosen 25cm DEM as the recommended DEM for this project because the data size is small and can be downloaded easily and it also provided a more detailed view of the ground features.
Task 2: Build DEM of Valley around Pats Cabin Reach from Lidar
A Lidar DEM was construct for the same reach to provide a little more landscape context and demonstrate the resolution differences between the raw survey data and Lidar. Click here for detailed instruction.
The differences between the DEM created from raw survey data and the Lidar DEM created with respect to channel change detection. Click image for larger view of this comparison.
Surveyed data:
- Cons:
- Collecting this data at one time point must be tiresome and time consuming.
- Having a repeat of these data point at the second time for a change detection analysis will as accurate as the first due to human error and limitation.
- Pros:
- It is more cost-effective than flying a airborne lidar.
- More accurate ground data can be obtained without the obstruction that can occur due to heavy vegetation covering some features when flying lidar.
- Higher resolution can be obtain from data collected.
Lidar data:
Pros:
- It is less stressful and data can over a large are in lesser period of time than manual survey .
- Easy data saving method and available through online sources.
- A exact repeat of data point capturing is easier for change detection analysis.
Cons:
- Some ground features may not be captured due to obstruction from vegetation.
- It is expensive.
This map illustrates a water depth DEM of Pats Cabin Reach using cross sections, and a longitudinal profile. Click image for larger view.
Task 3: Analyze DEMs to detect change in channel over time
The change in the channel was analyzed and a map of the areas of erosion and deposition was created. I also built a model tool to automate this change detection analysis process. Click here for detailed instruction.
Values less than +/- 20 cm were not included as we cannot be certain that they represent true change. This change analysis suggests that high flows led to sediment accumulation in upstream pond and partly downstream (blue region) due to blown out of the dams which removed sediment from the middle of the reach and deposited it downstream. Click image for larger view.
In this model, I used the 2010 and 2011 DEM data as inputs to calculate the change in elevation. While raster calculator does the subtraction, this gives me the initialdod. The initialdod was then classified using reclassified tool to obtain real values. The finaldod was then obtained by using the raster calculator ( multiplying the intialdod by the classified values obtained).
Reference
Reference
Joe Wheaton 2016, Lab 07 - Buiding DEMs. http://gis.joewheaton.org/assignments/labs/lab06-1/task-1. Retrieved on 15th March, 2017.
Open Topography Portal , http://opentopography.org/. Retrieved on 15th March, 2017.
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