Porta Farm Image Analysis. Summary. May 14, Lars Bromley, Summary... 1 Location... 4 Imagery... 4 Processing and Analysis...

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1 Porta Farm Image Analysis May 14, 2006 Lars Bromley, Summary... 1 Location... 4 Imagery... 4 Processing and Analysis...5 Summary According to Amnesty International the legal settlement at Porta Farm was destroyed in June Two QuickBird satellite images were obtained covering the area of Port Farm including a ëbeforeí image from June 22, 2002 and an ëafterí image from April 6, 2006 (see Figure One). The satellite images clearly show Porta Farm in 2002 while only some of its streets remain in Based on image analysis more than 850 structures were apparently removed during Operation Murambatsvina. More detailed samples (see Figures Two and Three) and background information is provided in the following pages. Figure One: Porta Farm Before and After Operation Murambatsvina Before: June 22, 2002 After: April 6, 2006 High resolution satellite imagery of the Porta Farm settlement in Zimbabwe. More than 850 structures are visible in the before (left hand) image, practically all of which are absent from the after (right hand) image. Note that the significant difference in color between the images is due to seasonality as the before image was captured during the dry season while the after image was acquired during the rainy season. Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 1

2 Figure Two: Porta Farm - June 22, 2002 Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 2

3 Figure Three: Porta Farm ñ April 6, 2006 Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 3

4 Figure Four: Porta Farm, Zimbabwe Location Porta Farm, Zimbabwe lay to the west of Harare, between Lake McIlwane and Lake Chivero and off the main Harare-Bulawayo road (Figure Four). Its coordinates were 30.8 East latitude and 17.9 South longitude. The settlement was about 1000 meters long and 250 meters wide and comprised of about 850 structures, three main roads, and numerous side streets (see Figure Two). Imagery Two QuickBird satellite images of Porta Farm were obtained from Digital Globe 1 for documenting home demolitions during Operation Murambatsvina. Specifically, one cloud free image of Porta Farm from June 22, 2002 was ordered from the Digital Global archives via the MapMart 2 company. This image from 2002 provided a before sample and clearly shows the Porta Farm settlement. Image characteristics are provided in Table One and a sample is found in Figure Two. Unfortunately, no suitable archive image of Porta Farm was available from Digital Globe for the period following Operation Murambatsvina. A new image task was ordered via MapMart so as to capture the site as it appears currently. This order was placed on March 20, 2006, and while a useable image was captured by QuickBird on April 6 it was not obtained by AAAS until May 12. The delay was due to the fact that the April 6 image was quite cloudy, and discussions with Digital Globe were necessary to obtain a high resolution sample in order to determine if the Porta Farm settlement was visible between the clouds. Image characteristics are provided in Table One and a sample is found in Figure Two. The QuickBird satellite carries a multispectral sensor measuring reflected electromagnetic radiation in the red, green, blue, and near-infrared wavelengths. Its spatial resolution is about 0.65 meters for the panchromatic product, making it suitable for assessing individual structures as small as a few meters on a side. It has a nominal overpass frequency of three days, though in practice about one image a week seems to be the most that can be expected. As with any spectral sensor it cannot penetrate clouds, further degrading its availability Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 4

5 Table One: QuickBird Image Characteristics Before: June 22, 2002 After: April 6, 2006 Catalogue ID Catalogue ID: 02JUN S2AS- 06APR S2AS_R1C _01_P _01_P002 Time of Acquisition 11:43 GMT Area: 25 square kilometers Coordinates nwlat = nwlong = selat = selong = Cost: $325 Time of Acquisition: 08:37 GMT Area: 64 square kilometers Coordinates nwlat = nwlon = selat = selon = Cost: $1792 Processing and Analysis The QuickBird Images were obtained in GeoTIFF format and imported into ERDAS Imagine for viewing. Both images are cloud free over Porta Farm, though significant clouds are found especially in the whole of the 2006 image. Following importation into ERDAS, subsets showing just the Porta Farm area from both the 2002 and 2006 images were extracted. Notably, as the two images represent two different seasons in Zimbabwe they are markedly different in color. The 2002 image was acquired in June, the middle of the southern Africa dry season, and thus the image shows relatively little vegetation. The after 2006 was acquired towards the end of the rainy season in April, and thus vegetation is relatively lush and the image is much more green than the 2002 image. Other than changing the overall appearance of the image this seasonal difference has no impact on the image quality. Visual analysis of the 2002 image was undertaken to determine the approximate number of structures removed during the interim period. Specifically, the 2002 image was loaded in ArcView GIS and individual visible structures were counted (see Figure Five). Total structures counted were 870. Dimensions of the settlements and its structures were also measured, indicating a long axis of 1017 meters and a short axis of 257 meters for Porta Farm. The size of structures themselves Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 5

6 varied quite widely, with small structures measuring 4 meters on a side and larger structures measuring up to 40 square meters and more. Figure Five: Porta Farm Structure Count Porta Farm Schematic of Structures The Porta Farm settlement. Center-points of structures in Porta Farm. Images (C)COPYRIGHT 2006 DigitalGlobe, Inc. ALL RIGHTS RESERVED. 6