[OSGeo Africa] Albers Equal Area projection datum WGS84

[Aslam Parker]

Hi

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.

We use the 1/6 rule (for Albers Equal Area Conic) to determine the standard parallels and minimise the distortion.
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).
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)
35deg-22deg = 13deg
1/6 of 13 deg = 2.1666666deg
So lat_1 = 22deg + 2.16666666 =24.166666
      lat_2 = 35deg – 2.1666666 = 32.8333333

The lon_0 is chosen as the centre of the project

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.







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From: Africa [mailto:africa-bounces at lists.osgeo.org] On Behalf Of Gavin Fleming
Sent: 31 July 2016 06:42 PM
To: africa at lists.osgeo.org
Subject: Re: [OSGeo Africa] Albers Equal Area projection datum WGS84

Hi Heidi

On 31/07/2016 17:51, Heidi Van Deventer wrote:

Dear all



I have an ArcGIS 10.3 geodatabase with feature classes in a custom projection of South Africa:

Albers Equal Area prj with WGS84 as a datum.
QGIS tries to match an incoming CRS with one it has on record and if it can't you'll have to provide one.

So, in Settings->CustomCRS, create one called, say 'Albers SA' with your custom parameters.

Here's one, for example, that I use when working with the whole of South Africa:



+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

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.

hth

Gavin






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.



Please advise how I can solve this problem - should I edit it in QGIS ?



Thanks in advance

Heidi







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