Polyconic projection of the Earth centered on meridian 180 degrees west, showing a bowed gap where east meets west.
I did a single polyconic projection of a Blue Marble image of the globe in an earlier article, comparing it to the sample projection in the proj (v.3) documentation. Although seemingly elongated, I called it a success. But when animated by reprojecting 360 times, varying the central meridian from -180° to +180°, I saw that something weird happens where the outside edges meet — there is sometimes, not always, a gap. I don’ t know if this is just an artifact of the projection that is to be expected, or if there’s a problem somewhere in my method or the software. You can see it in the picture at the right and in the animation below.
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Radar on Blue Marble image
This is a test run of the nation-wide (lower 48) weather radar animated on a Blue Marble image, with state outlines. The YouTube link is here, and it’s embedded below, along with a short discussion of the necessary steps to make it, and what could make it better.
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NOAA radar overlaid on a part of NASA's Blue Marble image of the southeast US.
The gdal set of GIS utilities is nothing new, and they are even incorporated into other GIS programs. But that does not mean that everyone who will ever need to master them has already done so, and it does not mean that mastering them is simple. It’s not really difficult to start using utlities like gdalwarp directly, but there are details that keep it from being really simple. This article is part of my notes to myself as I try to learn some of this.
I wrote an article about using gdalwarp to perform a pair of standard projections from a rectilinear view of the Earth using a Blue Marble image from NASA’s Earth Observatory. Below I discuss using a custom projection to do something marginally useful — combining the Blue Marble with some NOAA weather imagery.
Below are the steps that went into making that image. Note that I work in Linux, but Windows and likely Mac versions of these tools exist.
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Here are the results of some playing with gdalwarp and NASA’s Blue Marble images of the Earth. My experiences with gdalwarp and NOAA’s national weather radar mosaic, used to create a time-lapse animation of twelve months of weather, prompted a comment on the video’s YouTube page by someone having difficulty using gdalwarp with Blue Marble images. That prompted me to write up how I used my limited understanding of gdalwarp and related subjects for those radar images. But of course I had to try my hand on the Blue Marble next. What follows is how I went about it and the results I got.
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