Introduction

Quantitative estimation of building damages is extremely important after a disastrous event. An integrated remote sensing and GIS-based analysis approach can provide timely information on building damages. This preliminary analysis can help both national and international humanitarian actors to coordinate and plan aid response as well as need assessment operations. Remote sensing-based building damage assessment is generally performed using qualitative (e.g. photointerpretation) or quantitative methods (e.g. image classification). Pre- and post-event satellite images once acquired can be analysed and interpreted by expert analysts able to categorise assessed building according to different damage classes.

Tropical Cyclone Harold 20 made landfall on Luganville in Vanuatu on the 7th of April 2020, causing extensive damage to buildings and infrastructure.  Government decision-makers at National Disaster Management Office need damage statistics quickly to address emergency response needs.

In this exercise you are going to use a Very High-Resolution Satellite image taken on the 9th of April 2020 and Compare Against an Image from 07 September 2019, to detect the damages.

Pre- and Post-Disaster Very High-Resolution Satellite Imagery to detect damage in Buildings

Learning Objectives

By going through this exercise, you will learn to apply GIS methodologies and tools to:

  • Create a personal geodatabase for remote sensing-based building damage assessment
  • Perform building damage assessment using pre and post disaster aerial and satellite images (visual interpretation)
  • Generate a building damage intensity surface for assessed areas.

Data Input

X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Input

File NameData TypeSource
Raster\Lug_07Sep2019_WV2_R1C1.tifRaster (Post-Disaster)World View 2, Maxar
Raster\Lug_09Apr2020_WV2_R1C1.tifRaster (Pre-Disaster)World View 2, Maxar
Vector\Lug_AOI.shpVector (Polygon) 

Create a Personal Geodatabase for Data Storage

A Geodatabase is a data storage and management framework developed by ESRI. It is a relational database containing spatial and non-spatial objects. The primary elements of a Geodatabase are feature classes, rasters and tables. A Feature Dataset is a container for feature classes (point, line, and polygon) that are thematically related and share the same coordinate system and geographic extent. Geodatabases offer many advantages for GIS analysis such as: centralized data storage, support for advanced feature geometry, and more accurate data entry and editingthrough the definition and use of attribute domains.

For this exercise, you will create a personal geodatabase which will contain a point feature class corresponding to those building you will be assessing the damage level. To improve the overall quality of your dataset and to make the data entry process faster during the assessment you will create a number of different domain values to define a set of “legal attributes” which you will use to populate your geodatabase.

  • Open the ArcMap and connect to the folder from the catalogue paneX:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Output\Vector
  • Right Click on folder “Vector”> New > Personal Geodatabase
Creating new personal geodatabase
  • Type name as “TCHarold_Damage_Assessment”

Create Coded Value Domains

Attribute domains are rules that describe the legal values of a field type (a field will not accept a value that is not in that domain). Using domains helps ensure data integrity by limiting the choice of values for a field. To create new domain values:

  • Right click on “TCHarold_Damage_Assessment” geodatabase > Properties > Domains tab
  • Enter the Domain Name: Damage_Class
  • Type a Domain Description: Class of Damage
  • Select Short Integer as Field Type
  • Select Coded Values as Domain Type
  • Leave default values for Split and Merge Policy
  • Enter the following coded values and description:
    1 Possible Damage
    2 No Visible Damage
Creating Domains
  • Click apply
  • Repeat the same procedure to “Confidence_ID” domain
    1 Very High
    2 Medium
    3 Uncertain

Create a New Feature Dataset

  • Right click on “TCHarold_Damage_Assessment.mdb”> New > Feature Dataset
  • Name it as “Damage_Features”

The “New Feature Dataset Wizard” will ask you to select a coordinate system to set the spatial reference for the “Damage_Features” feature dataset:

  • Under Projected Coordinate Systems, go to UTM > WGS 1984 > Southern Hemisphere > WGS 1984 UTM Zone 58S
  • For Vertical Coordinate Systems, select “None” (no need to define a vertical coordinate system for this exercise)
  • Accept default values for XY Tolerance (0.001m is more than acceptable for the geodatabase you are about to create)
  • Click Finish

Create a New Feature Class

  • Right click on “Damage_Features” feature dataset > New > Feature Class
  • Enter “Building_Damage_Pointsas name
  • Select “Point” as feature type
  • Do not check any geometric properties > Click Next

We are going to create two attribute fields for capturing Damage Class and Analysis Confidence. Then both fields must be linked to the domains created in the previous section.

  • Create two new Fields “Damage” and “Confidence” and set the Data Type to short integer
  • Link appropriate domain to each field from the field properties section
  • Click Finish
Creating new feature class

Now you have a Geodatabase for performing damage analysis.

QUESTION

Why did we set the Damage field data type as short integer?
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Building Damage Assessment

Damage assessment maps are primary based on satellite analysis using pre- and post-disaster imageries. Photo interpretation methods are generally used to identify damages from imagery. Damage assessment maps aim to provide an overall view of affected areas, including a rapid assessment of the extent of damage. These maps are very useful for planning, coordination and setting priorities for both disaster relief and need assessment activities. A detailed inventory of all areas affected by disasters may require large-scale satellite derived mapping products for which high and very high-resolution imagery becomes necessary.

For this exercise, you will perform building damage assessment using a pre-defined Area of Interest (AOI) with Worldview 2 satellite image acquired after the TC Harold on the 9th of April 2020 and reference data from same satellite senor captured on the 07 September 2019.

  • Save the ArcMap project “M5_TCHarold_Damage_Assessment.mxd” in X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Workspace
  • Browse in the Data_Output\Vector folder and select the newly created feature class “Building_Damage_Points” inside the geodatabase “TCHarold_Damage_Assessment.mdb”.

This is the data layer (empty feature class) you will be editing during the damage assessment.

  • Load pre and post disaster images from the following location (Lug_07Sep2019_WV2.tif, Lug_09Apr2020_WV2.tif) X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Input\Raster

QUESTION

What is the spatial resolution of pre and post disaster images?
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Add symbology

  • Load “Lug_AOI.shp” from X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Input\Vector
  • Zoom to “Lug_AOI” and make sure that pre and post disaster images are both turned on
  • Right click on the “Buildings” > Properties > Symbology > Categories > Unique symbols to apply appropriate symbols
Add symbology
  • Click apply > Click OK

The above step will be super useful when we start digitising the buildings.

Digitise Point

  • Right click on the “Buildings” > Edit Features > Start editing. With this, editor menu will pop-up.
  • Click the create feature button
Editor Toolbar

A “Create Features” window will be visible where you have options to digitise both Damaged and Not damaged buildings

Create Features window
  • Activate the Effects toolbar
  • Use the Swipe tool from the Effects toolbar to switch compare post to pre disaster image to detect eventual building damage and the corresponding damage level
Effects toolbar
  • When you have identified a building and its damage status, click the correct symbol in the create feature window > Place it in the correct location in the map
  • Open the attribute table of the building to enter appropriate values
  • Save edits from editor menu

You will notice the “correct symbols” you used in the create feature window have appeared in the building damage attribute table.

Attribute table – new digitized points
Workflow for damage digitization

*TIPS

Now you can continue the above process to digitise all the damaged and not damaged buildings in the AOI. To do a fast assessment you can digitise all buildings from pre disaster imagery and add damage attributes in bulk.

Damage Intensity Surface Generation

Sometimes during post disaster damage assessments, we end up with points or polygons of different buildings, however this kind dataset cannot convey the information of damage distribution. It is a common practice to prepare damage distribution surface by interpolation or kernel density technique. Both methods have their limitations and it is up to the analyst to select appropriate technique.

In the first part of the exercise, you are going to prepare a damage intensity surface. In the second part, you will be making your own thematic map using damage intensity surface. The main challenge of this exercise is to figure what information you would like to convey with your map. We will also use symbology and labelling to convey this information in a clear, coherent, and efficient manner. The final output will be a pdf map that will be presented and discussed.

Interpolated Damage Intensity Surface

  • Zoom to the building damage point layer that has been created (AOI)
  • Open the attribute table of the “Building_Damage_Points” > Review the values in the field – “Damage_ID
  • In case you have not finished the previous exercise, you can open “Buildings_Damage_Assessment_Full.shp” in X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Input\Vector
  • Add a new Attribute Field. Go to Option > New Attribute Field: “Damage_Z”

The new “Damage_Z” attribute field will be used to obtain new integer values corresponding to the different damage level classes:
– No visible Damage (Damage_ID) = 0 (Damage_Z)
– Possible Damage (Damage_ID) = 10 (Damage_Z)

  • UseSelect by Attribute” to select the different damage classes for the attribute field “Damage_ID”

Use Field Calculator” to assign values listed above for “Damage_Z

Adding Field “Damage_z” and result after using the Field Calculator

Note that “Damage_Z” are values that are assigned to carry more importance towards damaged buildings and can be changed to match a specific damage scale as needed.

  • Go to ArcToolbox > Spatial Analyst toolbar > Interpolation
  • Choose Inverse Distance Weighted (IDW)
  • Make sure that the Building_Damage_Points data is selected in the input points window and that the Z value field has “Damage_Z” selected
  • Save output raster in X:\UNOSAT_ADV_FJI\Training_Material\M5\Practical\Data_Input\Raster
  • Run the tool with the OK button
IDW for damage assessment
  • After the tool has finished and added the model output raster to the TOC, open the Symbology tab in the Properties dialog box of the newly created raster
  • Change the Stretch Type to “Classified”
  • Select a colour ramp which would help easily show an increase in values (e.g. light to dark)
  • Review the output

QUESTIONS

Where would you say is the highest concentration of destroyed buildings based on the interpolation surface?
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DAMAGE INTENSITY USING KERNEL DENSITY FUNCTION (HEAT-MAP)

QUESTIONS

Try running kernel density function to demonstrate building damage intensity. Which result do you prefer & why?
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Kernel Density for damage assessment

ANNEX 1: Image Legend Key (Examples of Damage Classes)