Image Orthorectification using Erdas Imagine · Erdas Imagine. Digital air photo #3292 was chosen...

Image Orthorectification using Erdas ImaginePrepared By: Chen ShiDate: November, 2014Organisation: Center of Geographic Science

Overview of the Project

Discussion of Reference Data

Discussion of Methodology Discussion of QualityThe goal of this project is touse an existing orthorectifiedimage & digital elevationmodels to rectify an imagescene of Annapolis Royal,Nova Scotia using thePolynomial Model withinErdas Imagine.Digital air photo #3292 waschosen for this analysis (asyou can see in the left cornerof the picture on the left).Through this whole project,two major deliveables will becreated and they arefollowing: 1. Image3292convention file; 2. GCP3292convention file.

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Reference data: annapolis_north_2012_25cm_colour_subset.ecwThe projection used for thisreference data isUTM_Zone20_North. Spheroidis GRS 1980.The scale factor at centralmeridian is 0.999600. Longitudeof central meridian is63:00:00:000000 W, latitude oforigin of projection is0:00:00:000000 N. FalseEasting is 500000 meters andFalse Northing is 0.000000meters.Anna_1m_gnd.img (the digitalelevation model file) is alsoadded as a reference layer toimprove the accuracy of theimage orthorectification.

Image 3292 (input source) can be converted to real-world groundcoorrdinates by referencing the image toannapolis_north_2012_colour_subset.ecw (reference source).In order to accomplish this task, ground control points (GCPs) needto be selected from both the input source and the reference source,Such process is called georeferenc. The model used for this projectis Polynomial in Raster/Multispectral/Transform andOrthorectification.

A geographic coordiante system is used to assign geographiclocations to objects. In this project, the reference dataset(annapolis_north_2012_25cm_colour_subset.ecw) already has itsown projection and relevant coordinate system (Easting andNorthing). Therefore, the georeference process invloves assigningimage 3292 data to the reference map coordinates.

In the multipoint Geometric Correction window, 20 control points areslected to guarantee approximately one point in every imaginaey 100mgrid square. 5 check points in Image 3292 were placed well spread outand check if all the values in the error column is less than 1. Save theinput layer and reference layer once the check points are done andqualified.Click the Resample icon on the main toolbar to open the resamplewindow. After the new 3292_utm.img file is created, the user can start toverify the rectification process by turn on Pixel Transparency underMultiSpectral/View. Also, image can be ehnaced by Discretre DRA.

Georeference Model:

Coordinate System:

Resampling Technique:

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Problems Encountered:

RMS & Residual Results:

Visual Assessment of the Rectified Image3292

The overall process of this project went pretty well. However, I had ahard time in finding the relative locations from the input image to thereference map. The solution is quite simple by just rotate the imgaeto 90 degrees clockwise. But this step is quite easy to be ignored ifyou are doing the orthorectification for the first time.

20 control points were selected in both the input image andreference dataset. Also, 5 additional check points were created tomake sure that the total error is less than 1, which is a resonableresult. According to the results of image 3292 check points, themax error value is 0.750 and the lowest value is -0.141.

The overall Image3292_utm.tiff result is quite satisfied. However, onemisalignment has been identified through using the swipe tool in Edras.As shown in the pictures above, the edges of the lake are slightlymismatched. The Red circle in the pictures above are the control pointsselected for this orthorectification. The reasons for such misalignmentcould be a lot. Obviously, the input and reference images were taken indifferent time and therefore the water capacity for this two areas could bedifferent. Picture on right looks like a drought season compare to the oneon the left.

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Misalignment0 0.5 1 1.5 Miles

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0 0.5 1 1.5 Miles

Pic1: Location of Image 3292

Pic2: Reference Dataset: Annapolis_north_2012_25cm_colour

Pic 3: Multipoint Geometric Correction Dialogue

Pic 4: Image3292 before RotationPic 5: Image3292 after Rotation

Pic 6: Misalignment of the image

References: ERDAS Workbook 2; ERDAS Help Projection: UTM_Zone20_North, GRS1980.Disclamier: This is the student product and any changes made should be notified before.