Mosaicking Landsat Images Using ArcGISSTUDENT HANDOUT

Instructor Note: This exercise uses three complete Landsat 5 scenes. Approximately 1.5 gb of storage is required. If data storage or computer processing resources are limited, consider providing students with images containing fewer bands or spatially subsetting them using the Grand Canyon AOI shapefile.

Objective

In this exercise, students will learn how to combine multiple Landsat 5 scenes to create a seamless, color-balanced image of a large geographic area. They will learn the basic features of the ArcGIS Mosaic dataset tool to compile and enhance visually enhance the mosaic. Some functions of the Image Analysis tools will also be examined. Finally, they will learn how to export the mosaic to a permanent image file.

Background

Landsat image data is an important resource for many geospatial analysis projects. Analysis areas, however, often extend beyond the coverage of a single image. While multiple images can easily be viewed in ArcMap, variations in scene illumination and other radiometric properties often cause the images to appear inconsistent with one another, with areas masked by NoData areas of one image or sharp contrasts in tone or brightness between images. For this reason, images are often stitched together, or mosaicked, to provide a seamless, color balanced view of a project area. When done well, the final result of the mosaic process is an image comprised of multiple scenes that appears as though it was collected as a single image.

Developed 2014 by the Integrated Geospatial Education and Technology Training (iGETT) project, with funding from the National Science Foundation (DUE-1205069) to the National Council for Geographic Education. Opinions expressed are those of the author and are not endorsed by NSF. Available for educational use only. See www.igettremotesensing.org for additional remote sensing exercises and other teaching materials.

One drawback of a mosaic is that it can often result in a very large data file. For example, a single Landsat 5 scene, with all bands present, can exceed 250 mb. To aid with this file size issue, the ArcGIS Mosaic dataset tool creates a “virtual” mosaic. In setting the correct options and running the tool, the multiple images are displayed as single image without creating a new image file. Further enhancements can be performed using ArcGIS’s Image Analysis tools. If a new file is needed, the mosaic can then be exported and saved as a single file for future use.

Materials and Equipment

1. Windows PC Workstation2. ArcGIS 10.1 (Standard or Advanced) or higher with Spatial Analyst extension.3. Shapefile of Grand Canyon National Park boundary that can be used to subset the images prior to

mosaicking, if necessary.grand_canyon.shp

4. Shapefile of a rectangular area around Grand Canyon National Park (used to subset, if necessary)

grand_canyon_aoi.shp grand_canyon.shp5. Landsat 5 Image files covering Grand Canyon National

Park.lt50370342004289pac01.imglt50370352004289pac01.imglt50380352004280pac01.img

The .img file extension indicates these images are in the Erdas Imagine ™ file format. Erdas Imagine is one of several popular remote sensing analysis software programs. Like .tif, the .img file format is directly supported in ArcGIS and several other GIS software packages.

Instructor Note: Prior to the exercise, a folder named gc_l5 should be prepared, containing the shapefiles and images. The three Landsat 5 image files should be placed in a subfolder named images.

Student Requirements

1. Follow the instructions step by step. They will lead you through the exercise.2. Answer any questions in the spaces provided. These will be discussed during the exercise

review.3. If you have any system problems, ask your instructor for assistance.4. All work will be saved in your “student” directory. When naming files you create, always use the

suggested name and format. This will assist in the identification of these files later.

Student Tasks

Examine image files in ArcMap. Create a File Geodatabase to store the Mosaic Dataset. Create a Mosaic Dataset to define the contents of the Raster. Add images (rasters) to the Mosaic Dataset. Set options for the display of the mosaic. Export the Mosaic Dataset to a new image file.

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PART 1. Explore the Data

a. Open a new ArcMap documentb. Add the Landsat scenes contained in the

gc_l5/images folder.c. Add the Grand Canyon NP Boundary

shapefile. Make it hollow with an outline color that shows up against the image background.

d. Examine the three images.a. Further improve the appearance of the

images by changing the band combination being viewed from 1, 2, 3 to 5, 4, 2 (a short wave infrared composite).

Right-click on each image in the Table of Contents and select the Symbology tab.

Change the order of the bands from top to bottom to 5, 4, 2.

Press OK. Check the Display

backgroiund Values (R, G, B) box, leave the default settings.

b. Note that the edges of the images are still apparent and that the color balance among the images is not consistent.

The two easternmost images align and blend quite well. These two images were collected on the same day, on the same pass, within seconds of one another. Atmospheric and illumination conditions were nearly identical at the time each image was collected.

What are some considerations one should keep in mind when selecting images for use in a mosaic?________________________________________________________________________________________________________________________________________________________________________________________________

e. Remove the three Landsat scenes from the view.

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PART 2 – Mosaic the Landsat 5 Scenes

Step 1. Create The Mosaic Dataset.

Now you will create a file Geodatabase to store the Mosaic Dataset, then create the Mosaic Dataset.

“A mosaic dataset allows you to store, manage, view, and query small to vast collections of raster and image data. It is a data model within the geodatabase used to manage a collection of raster datasets (images) stored as a catalog and viewed as a mosaicked image.” ArcGIS Desktop HelpImages are not stored in the Mosaic Dataset, instead the dataset contains pointers to the image files, allowing them to e portrayed as a single, seamless image, “on-the-fly.”1. Open ArcCatalog within ArcMap.

2. Browse to and right-click on the gc_l5 folder 3. Select New -> File Geodatabase. A new, empty File

geodatabase will appear in the gc_l5 folder. Name the new file geodatabase gc_l5.

4. Right-click on the gc_l5 File geodatabase. Select New -> Mosaic Dataset… The Create Mosaic Dataset dialog will appear.

5. In the Create Mosaic Dataset dialog, enter the following:

a. Output Location: \gc_l5\gc_l5.gdbb. Mosaic Dataset Name: gc_l5_mosaicc. Coordinate System: Browse to and select

WGS_1984_UTM_Zone_12Nd. Select OK.

6. The Mosaic dataset is created and empty “placeholder” files for the images, their footprints, and boundaries appear in the Table of Contents.

A mosaic dataset consists of: A catalog that provides the

source of the pixels and footprints of the rasters

A feature class that defines the boundary

A set of mosaicking rules that are used to dynamically mosaic the rasters

A set of properties used to control the mosaicking and any image extraction

A table for logging during data loading and other operations Optionally, a seamline feature class for seamline mosaicking

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Optionally, a color correction table that defines the color mapping for each raster in the raster catalog

(ArcGIS Desktop Help)

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Step 2. Add Images To The Mosaic Dataset.

The images files will now be referenced to the Mosaic Dataset. There are several methods for doing this; however, images formatted as GeoTIFFs or img files work best when placed in a folder that can be referenced during the mosaic process.

1. In ArcCatalog, right-click on the new gc_l5_mosiac Mosaic Dataset.

2. Select Add Rasters… The Add Rasters to Mosaic Dataset dialog will appear.

3. In the Add Rasters to Mosaic Dataset dialog, enter the following:

a. Mosaic Dataset: \gc_l5\gc_l5.gdb\gc_l5_mosaic

b. Raster Type: Raster Datasetc. Input Data: Workspace. Browse to and

select the gc_l5/images folder. d. Check the box next to Update

Overviews.e. Expand Advanced Options.

i. Check all the check boxes.ii. Select

OVERWRITE_DUPLICATES in the Add New datasets Only (optional) menu.

iii. Enter Landsat 5 Mosaic in the Operation Description box.

4. Select OK.5. Boundary, Footprint, and Image layers are

added to the Table of Contents and a rather unattractive version of your images appears in the Data View. (You may have to zoom in to see the images clearly.) You will fix the appearance of the image in the next step.

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Step 3. Improve the Appearance of The Mosaic Dataset

In this step, you will improve the appearance of the mosaic to make it appear seamless, in a useful band combination, and properly color balanced.

a. In ArcCatalog, right-click on the gc_l5_mosaic mosaic dataset.b. Select Modify –> Define NoData… The Define Mosaic Dataset NoData dialog will appear.c. In the Define Mosaic dataset NoData

dialog, enter the following:a. Mosaic Dataset: \gc_l5\

gc_l5.gdb\gc_l5_mosaicb. Leave the query definition box

bland and the Number of bands as 7.

c. In the Bands for NoData Value box, use the pull-down to select ALL_BANDS. Enter 0 in the NoData Value column.

d. Select OK. The

appearance is somewhat improved.

d. Once again, right-click on the gc_l5_mosaic mosaic dataset in the catalog. Select Enhance -> Color Balance.

e. In the Color balance Dialog, set the Balancing Method to DODGING and the Color Surface Type to SINGLE_COLOR.

f. Select OK.g. In the Table of Contents, right-click on the Image layer and select properties.

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h. Select the Mosiac tab and change the Mosaic Method to Closest to Center and the Mosaic Operator to Blend. Make sure the Order Ascending box is checked. Select OK.

i. Depending on the order of the images in the display, you may have to experiment with other settings to get the results you desire.

j. Depending on the setting you used and your zoom level, you may see some artifacts of the edges of the images used to create the mosaic. These usually disappear when zoomed in.

k. Notice how the order of appearance of the images changes with different zoom levels. This order is also affected by the settings in the Mosaic tab of the Layer Properties. Experiment with these settings and notice how the appearance of the images changes.

Seamlines can be used to control the order of appearance of the images. The settings are similar to those used in the Mosaic tab described above, but give additional options for refining the appearance of the mosaic. For more information on Seamlines, consult ArcGIS Desktop Help.

l. Further improve the appearance of the image by changing the band combination being viewed from 1, 2, 3 to 3, 2, 1 (a natural color composite).

a. Right-click on Image in the Table of Contents and select the Symbology tab.

b. Change the order of the bands from top to bottom to 3, 2, 1.

c. Change the Stretch type to Standard Deviations.

d. Press OK.m. The appearance may be improved a little

more by selecting the Mosaic tab in the Image’s layer properties and changing the settings so that Mosaic Method is By Attribute and the Order Field is Center Y. This should send the image with the clouds to the back and image edges are less obvious.

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n. Improve the brightness and contrast using the Image Analysis tool.a. Select Windows -> Image Analysis from the main menu.b. The Image Analysis tools will appear.c. Select the gc_l5_mosaic item in the Image Analysis dialog.d. Zoom and experiment with the sliders and other items in the Display portion of the Image

Analysis dialog.e. Experiment with different band combinations. You will have to reset the Display options

each time you change band combinations.

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Step 4. Export Mosaic Dataset to a New File.

The Mosaic Dataset is useful for obtaining a fairly seamless view of multiple images; however, a more polished final product can be created by exporting the Mosaic dataset to a new image file. This reduces or eliminates the appearance of image edges and results in a single, color-balanced image.

1. Right click on Image in the Table of Contents.2. Select Export Data.3. In the Export Mosaic dataset dialog, select Mosaic Dataset (Original) for both the Extent and

the Spatial Reference.

4. Accept the defaults for the Output Raster.

5. Save your output as gc_l5_mosaic.tif in the gc_l5 folder.

6. Select Save. Select Yes in the Pixel Depth dialog.

7. When prompted, add your new image to the Data View. Zoom, pan, and explore the various band combinations and Display options just as you did with the Mosaic Dataset.

This may take a long time to process. Use your waiting time productively to learn more about Grand Canyon National Park by visiting the National Park Service’s Grand Canyon website at http://www.nps.gov/grca/index.htm /

Note: If you are still getting edges and seams, try experimenting with the Mosaic settings in the Image Layer properties.

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What are some useful applications of Mosaic datasets and mosaics?

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Congratulations! You have combined several Landsat images to create a single, seamless view of your project area. This is one of the most useful image processing techniques employed by geographic information specialists.

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Sources: ArcGIS Desktop Help, Copyright(C) 1995-2012 Esri.ArcGIS Raster Tutorial, Copyright © 1995-2010 Esri. All rights reserved.Images: USGS EarthExplorer Landsat Archive, http://earthexplorer.usgs.gov/. Accessed Dec 20, 2013.Grand Canyon National Park Boundary: Current Administrative Boundaries of National Park System Units 09/11/2013, shapefile downloaded from http://www.nps.gov/gis/data_info/, Dec 20, 2013.

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