Effect of Changing Datums, Projections, Resolution


   

We start with a DTM on the New Zealand horizontal and vertical datums.  The peak is at 162.0 m.





GDAL warp to WGS84 horizontal, EGM2008 vertical.  The "features" every 50 m horizontally and vertically come from GDAL.  The peak is now at 162.3.  Because there have been shifts both horizontally and vertically, the shapes of the contour lines have changed.





Resampling by mean aggregation to 30 m in red (square pixels), and 1 arc second in green (rectangular pixels, 24.8x30.8 m), with 2 m contours.  The centroids of the pixels shown with square symbol labelled with the elevation.  Peak in the 30 m DEM is at 159.7 m, with that of the 1" DEM also at 159.7 m but displaced to the NE.  The peak can only be at a pixel centroid; the distance between nearest centroids varies throughout the DEM.  Resampling drives elevations toward the mean, dropping peaks and ridges and raising valleys.


Contours from the 1 m DEM overlaid in blue, which should much more detail.  The closed-contour peak is closer closer to the 1" DEM (green) compared to the 30 m DEM (red), which is just a random effect from how the two grids interacted.  At this scale the 1 m contours are smooth, which the other two clearly show the individual line segments.



Reprojection and contour lines from GDAL.

Aggregation and map displays from MICRODEM.

GDAL transforms EPSG 2193+1169 (1 m resolution) to  EPSG 32769+3855 (1 m resolution).

MICRODEM mean aggregation from the 1 m to EPSG 32769 + 3855 (30 m) and separately EPSG 4326 + 3855 (1 sec)


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