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Background

I am facing difficulty in selecting an appropriate equal-area map projection suitable for the geographical extent of Southeast Asia. The main resource I am consulting is Savric et al's paper in 2016 titled 'Projection Wizard – An Online Map Projection Selection Tool' and the accompanying web-based tool (https://projectionwizard.org/). The geographical extent of Southeast Asia is 92.1892776489258949,-11.0097208023069015 : 141.0117645263674149,28.5432605743409908 (in decimal degrees). I plotted a similar extent at Projection Wizard, and the results show that only one projection is suitable;


Projection details

Equal-area projection for regional maps with an east-west extent

Cylindrical equal-area PROJ.4

Standard parallel: 00º 00' N

Central meridian: 116º 06' E


My question

However, I am not entirely sure how to use this result. It sounds like this means I will have to create my own custom projection (using the standard parrallel and central meridian identified by Projection Wizard) in PROJ4 program, which I am not familiar with yet.

Is there an EPSG projection equivalent that achieves similar results?

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    Personally, I don't like the projection wizard tool. A query to the EPSG registry database by ProjectedCRS with the word Asia in the name returns the EPSG:8859 system. Do you think that it is ok for you? i.stack.imgur.com/EsKWC.png Jun 6 '20 at 1:46
  • Thank you @GabrielDeLuca. It might be a suitable projection for me. But I have one question; on the EPSG page for EPSG:8859, under scope it says 'Scope: Very small scale equal-area mapping centred on Asia-Pacific.'. What does small scale mean? I may be mistaken but interestingly, EPSG:8859 (WGS 84 / Equal Earth Asia-Pacific) was created by the same people who created the projection wizard tool. But that information is irrelevant. Jun 9 '20 at 2:05
  • I like the people that created the Projection Wizard tool, I just personally don't like the tool. Small scale is... a small scale, 1/500000 is a smaller scale than 1/50000, think in it as zoom, it is like a small zoom. Jun 9 '20 at 5:04
  • Thank you. I can find EPSG:8859 on the EPSG registry database, but for some reason, it is not in spatialreference.org/ref. This means that although I can apply the projection in ArcGIS (where it can be found), I cannot perform any additional geoprocessing outside of ArcGIS, e.g. in R. This is because I cannot get the Proj4 syntax for EPSG:8859. Jun 9 '20 at 12:32
  • C:\>projinfo EPSG:8859 PROJ.4 string: +proj=eqearth +lon_0=150 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs +type=crs Jun 9 '20 at 20:43
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The wizard suggests the Lambert equal-area cylindrical projection whose distortion does not change with the choice of central meridian. The projection is a cartographically sound choice for your area, and thanks to the independence to the central meridian, you can use any global system that uses the projection in normal form with a zero standard parallel. The proj4 projection string is +proj=cea.

Unfortunately, the only EPSG system that contains CEA seems to be EPSG:6933 (WGS 84 / NSIDC EASE-Grid 2.0 Global), which uses stanadard parallels (latitudes of true scale) of +-30°, resulting in somewhat more distortion if you want to stick to standard systems only.

If you decide to proceed with your own projection, it is quite easy to modify the definitions from above to use zero latitude of true scale (in Proj4 and OGC WKT respectively):

+proj=cea +lon_0=0 +lat_ts=0 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

PROJCS["Your Projection Name",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],
            TOWGS84[0,0,0,0,0,0,0],
            AUTHORITY["EPSG","6326"]
        ],
        PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9108"]],
        AUTHORITY["EPSG","4326"]
    ],
    PROJECTION["Cylindrical_Equal_Area"],
    PARAMETER["standard_parallel_1",0],
    PARAMETER["central_meridian",0],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["Meter",1]
]
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  • Thank you @Jan Šimbera. May I ask if it is common practice to proceed with my own projection, as opposed to using standard systems? My experience thus far with projections is to use several UTM projections, which are not suitable for my case now. Secondly, for your suggestion of Proj4 sytnax, would it not include information on Central meridian: 116º 06' E? I reckon you are trying to explain that the choice of central meridian does not matter, but how would I change the Proj4 syntax to include it? Jun 9 '20 at 12:38
  • Actually, I have just realised that the Proj4 syntax you have suggested is the Proj4 syntax for Behrmann's projection which is supposedly an ellipsoidal (Lambert) cylindrical equal area with standard parallels at +/-30. However, I am now slightly confused as you mentioned the Proj4 syntax you suggested is not using the standard parrallels of +/-30. Jun 9 '20 at 12:48
  • Using a nonstandard system is AFAIK considered OK for cartographic purposes (making a specific map), not so much for maintaining and publishing general data. The projection I show differs from Behrmann by the +lat_ts=0 setting (Behrmann, used in EPSG:6933, has +lat_ts=30) that minimizes distortion at the equator while making it huge in polar regions. Jun 19 '20 at 17:19

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