Ground control points were captured in the field as described in the metadata record "Global change, biodiversity and conservation in terrestrial and coastal ecosystems on Heard and McDonald Islands [ASAC_1181]". The aim of this project was to identify the control points on various satellite images (AADC Image IDs 253, 267, 446 and 447) to determine image coordinates for the control points. These would then allow the satellite image to be adjusted to the ground control. There is a shapefile for each image with the features in the shapefile moved to the corresponding image location. Attributes in the shapefile include image coordinates (UTM43) and control point coordinates.
Ground control points were captured in the field by Dr Jenny Scott using differential GPS as described in "Global change, biodiversity and conservation in terrestrial and coastal ecosystems on Heard and McDonald Islands [ASAC_1181]". David Smith of the AAD Data Centre provided a shapefile called control_pts_UTM43 which had been exported from the differentially corrected GPS data. ... Two fields were added to the attribute table - UTM43_X and UTM43_Y, these were populated with the X and Y coordinates of the points in the UTM zone 43S projection of the shapefile. One copy of the shapefile was created for each image set: Control2005_07 shapefile was adjusted to AADC Image ID 446 Control2005_09 shapefile was adjusted to AADC Image ID 447 Control2004 shapefile was adjusted to AADC Image IDs 266 and 267 Control2003 shapefile was adjusted to AADC Image ID 253
The control points were identified on various satellite images (AADC Image IDs 253, 267, 446 and 447). Points were initially moved on the 2003 image, as this had been used in the field and the points marked on a 1:5000 hardcopy. The Description field in the attribute table, plus memory of the point's location in the field, plus the hardcopy, were relied on for accurate point location. The fact that Dr Scott did both the work plus the fieldwork was an important factor in achieving maximum accuracy. The most convenient working scale for positioning the points was 1:1900 (anything larger became too pixelated), with a larger scale used occasionally to check actual distance measurements.
Fields added to the attribute tables of the four new shapefiles:
'Moved' field (integer)
0 Point not moved 1 Point moved
'Comments' field (text)
Moved. Not moved, obscured by cloud. Not moved, not on image.
'Confidence' field (integer)
0 Point not moved 1 Point moved. Moderate accuracy due to deep shadow, haze, variations between images, or difficulty in pinpointing exact location (accuracy probably within 2-3 m). 2 Point moved. Greatest accuracy (probably within 1-2 m or less).
Image_No field The AADC image number from which the control point was identified
Platform field The satellite platform used to capture the image
Capt_date field The date the image was captured
Image_X field The X coordinate of the moved control point feature in the corresponding image coordinate system
Image_Y field The Y coordinate of the moved control point feature in the corresponding image coordinate system
UTM43_X field The original X coordinate of the control point before it was moved
UTM43_Y field The original Y coordinate of the control point before it was moved