<html><body name="Mail Message Editor"><div><br></div>Corrado:<div><br></div><div>My baser instincts here say not to even worry about the ellipsoid, given the data accuracy you're talking about. I'm not sure I understand quite what you mean by 20km x 20km, though. That is the most detailed data you will work with?</div><div><br></div><div>What will be the scale of your map? Something that fits in the journal pages?</div><div><br></div><div>Regards,</div><div>— daan Strebe</div><div><br></div><div><br></div><div>On Dec 9, 2008, at 1:24:17 AM, "Corrado Topi" <ct529@york.ac.uk> wrote:<br><blockquote style="padding-left: 5px; margin-left: 5px; border-left-width: 2px; border-left-style: solid; border-left-color: blue; color: blue; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="width: 100%; "><div id="felix-mail-header-block" style="color: black; background-color: white; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: silver; padding-bottom: 1em; margin-bottom: 1em; width: 100%; "><table border="0" cellpadding="1" cellspacing="1" width="100%"><tbody><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>From:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span title=""Corrado Topi" <ct529@york.ac.uk>">"Corrado Topi" <ct529@york.ac.uk></span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>Subject:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span style="font-weight: bold; ">[Proj] Re: Mapping Africa in an equal area projection</span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>Date:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span>December 9, 2008 1:24:17 AM PST</span></td></tr><tr><td width="70px" style="font-family: 'Lucida Grande'; font-size: 8pt; color: gray; text-align: right; vertical-align: top; font-weight: bold; "><span>To:</span></td><td style="font-family: 'Lucida Grande'; font-size: 8pt; color: black; text-align: left; vertical-align: top; padding-left: 5px; "><span title="proj@lists.maptools.org">proj@lists.maptools.org</span></td></tr></tbody></table></div><div id="felix-mail-content-block" style="color: black; background-color: white; width: 100%; "><div style="font-family: monospace; color: black; background-color: white; font-size: 8pt; ">Dear Daan,<br><br>the results will be shown on maps of Africa as a whole. That is we will show<br>maps of Africa, with different measurements and sample, but not of smaller<br>or larger regions.<br><br>We will use the maps in grass, and do some sample conversion using cs2cs /<br>proj.<br><br>The resolutions we will be working at are: 100 km x 100km initially, to<br>explore the data, 50 km x 50 km or 20 km x 20 km later on, but only if our<br>data are so good as to allow us to do so (we are not sure yet). The results<br>will be shown on a Africa - wide map in any case, and not on sub regions.<br><br>We will not use any larger or smaller scale maps.<br><br>What do you think?<br><br>Best,<br><br>strebe@aol.com wrote:<br><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Corrado:<br>><span class="Apple-converted-space"> </span><br>> You did not indicate if you will be making a single map of all of Africa,<br>> or a series of much larger-scale maps. If it is a map of all of Africa,<br>> then your choice of ellipsoid is not going to matter much, in which case<br>> WGS84 is a good default because GPS data sources are already referred to<br>> the WGS84 datum and because datum conversion parameters normally are<br>> specified against WGS84.<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Otherwise, use the ellipsoid and datum from which your source data are<br>> culled unless you have some specific reason not to. If your data come from<br>> many datums and ellipsoids, then again, it surely makes most sense to use<br>> WGS84 as a standard.<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> If you will be making a series of large-scale maps of (known) high<br>> accuracy, then it makes more sense to standardize on a datum that fits<br>> Africa better than WGS84. The Clarke 1880 ellipsoid dominates the extant<br>> African datums, but I am not aware of any named datum that has been<br>> formulated as a best fit for the entire African continent, though there<br>> are mean solutions for large regions, such as ARC 1950.<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> If your project is such a series of large-scale maps, by the way, I would<br>> not use an equal-area projection if I were you. A conformal projection<br>> will give more benefits on the ground and the areal inflation will be<br>> negligible until you reach medium-scale.<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Regards,<br>><span class="Apple-converted-space"> </span><br>> — daan Strebe<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> -----Original Message-----<br>> From: Corrado Topi <ct529@york.ac.uk><br>> To: proj@lists.maptools.org<br>> Sent: Mon, 8 Dec 2008 2:50 am<br>> Subject:=2<br>> 0[Proj] Re: Mapping Africa in an equal area projection<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Thanks Daan!<br>><span class="Apple-converted-space"> </span><br>> What ellipsoid / datum, would you choose? WGS84, ETRS89, or waht else?<br>><span class="Apple-converted-space"> </span><br>> Regards<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> strebe@aol.com wrote:<br>><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> Corrado:<br>>><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> Of the candidates you list, I would call Mollweide inappropriate because<br>>> it stretches Africa north-south to in order to reduce distortion<br>>> elsewhere. It really is best left for world maps, where that compromise<br>>> results in some benefit. Any of the others you list could be reasonable,<br>>> depending on how they are constructed.<br>>><span class="Apple-converted-space"> </span><br>>> Because Africa spans roughly<br>>> the same latitudinal range north as south, an equatorial Lambert<br>>> azimuthal equal-area is a fine choice, and will give you the lowest<br>>> distortion of the group. Nor are there any better projections to<br>>> recommend without going exotic. Sinusoidal is not a common modern choice,<br>>> and I do not recommend it in this case because you will end up with<br>>> higher distortion around the periphery than the other projections, and<br>>> with no compensation.<br>>><span class="Apple-converted-space"> </span><br>>> Unless your application would benefit from straight parallels and<br>>> meridians, or unless your two standard parallels happen to be exactly<br>>> latitudes of opposite sign, then Albers will always perform better than<br>>> cylindric equal-area. But the advantages of Albers are largely lost on<br>>> Africa unless you were willing to do something exotic like a heavily<br>>> oblique aspect. Left with its standard parallels set to the graticule<br>>> parallels, it would end up very<br>> similar to the cylindric equal-area<br>>> (because the two standard parallels would be nearly latitudes of opposite<br>>> sign), and in that case it cannot approach Lambert for low distortion<br>>> across your region of interest.<br>>><span class="Apple-converted-space"> </span><br>>> Regards,<br>>> — daan Strebe<br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br><br>--<span class="Apple-converted-space"> </span><br>Corrado Topi<br><br>Global Climate Change & Biodiversity Indicators<br>Department of Biology, University of York, UK<br><br>_______________________________________________<br>Proj mailing list<br>Proj@lists.maptools.org<br>http://lists.maptools.org/mailman/listinfo/proj<br><br></div></div></div></span></blockquote><br><div><br></div></div><div class="aol_ad_footer" id="u0BCDD18F8AB248DD99F5504D4F6243A6"></div></body></html>