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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">A bit of advice when using Albers Equal Area Conic Projection (with two standard parallels/SP’s). It is ideal for small scale political maps of the country
(where area of provinces are in proportion) and any instance where accurate area is required. NGI uses this projection for the 1:2200 000 National Map of South Africa and its provincial maps.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">We use the 1/6 rule (for Albers Equal Area Conic) to determine the standard parallels and minimise the distortion.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The standard parallels are chosen 1/6 south of the northern extreme of the project (lat_1) and 1/6 north from the southern extreme (lat_2).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">For South Africa, using 22 deg and 35 deg as rough approximations (for simplicity) of the northern and southern extremes of continental SA (don’t forget about
Marion and Prince Edward Islands)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">35deg-22deg = 13deg<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">1/6 of 13 deg = 2.1666666deg<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">So lat_1 = 22deg + 2.16666666 =24.166666<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> lat_2 = 35deg – 2.1666666 = 32.8333333<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The lon_0 is chosen as the centre of the project
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">The above example demonstrates the rationale behind the choice of SP’s. Users are advised to change the SP’s using the above rule if project is other than continental
South Africa, eg provincial , maps or maps of other countries.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext"> Africa [mailto:africa-bounces@lists.osgeo.org]
<b>On Behalf Of </b>Gavin Fleming<br>
<b>Sent:</b> 31 July 2016 06:42 PM<br>
<b>To:</b> africa@lists.osgeo.org<br>
<b>Subject:</b> Re: [OSGeo Africa] Albers Equal Area projection datum WGS84<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi Heidi<br>
<br>
On 31/07/2016 17:51, Heidi Van Deventer wrote:<o:p></o:p></p>
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<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<pre>Dear all<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>I have an ArcGIS 10.3 geodatabase with feature classes in a custom projection of South Africa:<o:p></o:p></pre>
<pre>Albers Equal Area prj with WGS84 as a datum.<o:p></o:p></pre>
</blockquote>
<p class="MsoNormal">QGIS tries to match an incoming CRS with one it has on record and if it can't you'll have to provide one.
<br>
<br>
So, in Settings->CustomCRS, create one called, say 'Albers SA' with your custom parameters.
<br>
<br>
Here's one, for example, that I use when working with the whole of South Africa:<br>
<br>
<br>
<o:p></o:p></p>
<p style="margin:0cm;margin-bottom:.0001pt">+proj=aea +lat_1=-24,241886157 +lat_2=-32,715290215 +lat_0=0 +lon_0=24,670537409 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs<o:p></o:p></p>
<p class="MsoNormal"><br>
Then choose it when you load those layers or, after you've loaded them, go the General tab in their layer properties and set the CRS to your new custom CRS.
<br>
<br>
hth<br>
<br>
Gavin<br>
<br>
<o:p></o:p></p>
<pre><o:p> </o:p></pre>
<pre><o:p> </o:p></pre>
<pre>When I try to open this in QGIS 2.8, I get an error message that two of the feature classes' coordinate systems aren't recognised, and it therefore does not display at all.<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Please advise how I can solve this problem - should I edit it in QGIS ?<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Thanks in advance<o:p></o:p></pre>
<pre>Heidi<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre><o:p> </o:p></pre>
<pre><o:p> </o:p></pre>
<pre>--<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>This message is subject to the CSIR's copyright terms and conditions, e-mail legal notice, and implemented Open Document Format (ODF) standard. <o:p></o:p></pre>
<pre>The full disclaimer details can be found at <a href="http://www.csir.co.za/disclaimer.html">http://www.csir.co.za/disclaimer.html</a>. <o:p></o:p></pre>
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<pre>_______________________________________________<o:p></o:p></pre>
<pre>Africa mailing list<o:p></o:p></pre>
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<p class="MsoNormal" align="center" style="text-align:center">Gavin Fleming - Joint MD - PrGISc [PGP1234]<o:p></o:p></p>
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