{"id":1947,"date":"2018-03-22T09:39:51","date_gmt":"2018-03-22T09:39:51","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=1947"},"modified":"2023-05-07T03:35:27","modified_gmt":"2023-05-07T03:35:27","slug":"taveuni-an-island-torn-in-two","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/taveuni-an-island-torn-in-two\/","title":{"rendered":"Taveuni – An Island Torn in Two"},"content":{"rendered":"

Spatial data of dragonfly observations from the Fijian island of Taveuni posed some interesting mapping problems .\u00a0 This post covers how we resolved them using a new projection.
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You may recall an earlier post where we talked about collecting dragonfly sightings across the Pacific.\u00a0 My colleague, Milen, is quite the dragonfly aficionado and has been traveling the Pacific building up a database of observations.<\/p>\n

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Earlier we set up a webmap<\/a> that he could use on his phone to collect locations and photos without a 4G or wireless internet connection and have them later synced to our GIS server here at Lincoln.\u00a0\u00a0 So far this has worked really well and he has used it on other islands, including the lovely Fijian Island of Taveuni.\u00a0 Here’s a bit of a map to give you an idea of where it is:<\/p>\n

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There could be worse places to have to collect data and many times I have offered to carry his bags, but so far, no luck…<\/p>\n

Anyway, after he had done his field work and the data were all synced, I got to work on putting together a map for Milen’s report.\u00a0 So I dutifully opened ArcMap and added the data – but something looked immediately wrong:<\/p>\n

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All I can see are two points!\u00a0 What’s going on here?\u00a0 Check out the scale of the map – 1:156,000,000+!\u00a0 At that scale I must be way way zoomed out so I’ll zoom in to the one of the left side to see what’s going on:<\/p>\n

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Definitely have some points there – a healthy 58 in total as shown in the table, but after a quick count only 49 of them here.\u00a0 Let’s zoom to the other side:<\/p>\n

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The remaining nine are over here.\u00a0 What’s the story?\u00a0 To help figure this out, I’ll add a basemap:<\/p>\n

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Well, this starts to explain things – my points are at opposite sides of my basemap!\u00a0 Let me zoom in again on the left hand side:<\/p>\n

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Poor Taveuni!\u00a0 It is an island rent in two by well meaning cartographers!\u00a0 Separated at birth at opposite ends of the world!\u00a0 Here’s the other side:<\/p>\n

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What we’re really dealing with here is a coordinate system problem.\u00a0 If you’ve been paying attention to the screenshots you might have seen the coordinates in the lower right hand corner:<\/p>\n

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These are clearly longitude and latitude coordinates in decimal degrees<\/a>.\u00a0 This comes from the fact that Milen’s phone used its on board GPS receiver to collect the points and the coordinate system GPS speaks is latitude and longitude.\u00a0 Lat\/Long is based on a roughly spheroidal planet and, when coupled with an elevation, is a 3D system.\u00a0 We will always face the fundamental problem of how to translate 3D spheroidal coordinates to flat 2-dimensional maps – that’s where projections come in.\u00a0 We’ve covered projections<\/a> ad nauseum previously but in essence, projections allow us to take 3D coordinates and translate them to a flat 2D map.\u00a0 If you think about that wall map that you had on your wall as a kid (maybe it’s still there?), it had a top and bottom and two sides (duh!).\u00a0 Most of those maps were probably centred horizontally on Greenwich as the prime meridian.\u00a0 This means that some places will end up on the edges, like Taveuni, and be split in two.\u00a0 That’s the case here.<\/p>\n

So how do I deal with this?\u00a0 What I need is a different projection that centred on the Pacific.\u00a0 The basemap projection is known as WGS 84 (World Geodetic System 1984<\/a>).\u00a0 After a bit of searching I managed to find one – WGS 84 PDC Mercator (The PDC stands for Pacific Disaster Center<\/a>, who developed this projection).\u00a0 This is a projection and accompanying coordinate system that is Pacific centred so should reunite the opposite ends of Taveuni.<\/p>\n

Not to get too complicated, but there are two ways I can move forward: I could either just change the coordinate system of the map (from Layers > Properties > Coordinate System), or I could be a bit more thorough and “project” the data (transform its coordinate system) to the new coordinate system.\u00a0 This means creating a new layer with the projection set to PDC Mercator.\u00a0 Here’s how:<\/p>\n

From ArcToolbox > Data Management Tools > Projections and Transformations I chose the Project tool.<\/p>\n

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The input file is my original data layer.\u00a0 The tool automatically picks up the coordinate system from the data themselves.\u00a0 I’ve specified a new output layer called “TaveuniObservationsPDCMercator.shp”.\u00a0 The new coordinate system can be found at Projected Coordinate Systems > World > WGS 84 PDC Mercator.\u00a0 In some cases, specific geographic transformations are needed (such as going from NZMG to NZTM) and if one is needed, you’ll see some options under that box.\u00a0 One is not needed here so we can click OK and create a new layer.\u00a0 I’ll add that to a new, blank map and:<\/p>\n

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Now my points are more in the centre of the map and look to be in the right place.\u00a0 Zooming in, I can imagine many happy Taveunians:<\/p>\n

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Now that’s more like it.\u00a0 Families reunited.\u00a0 Huge fuel savings in getting from one end of the island to the other.\u00a0 I can now carry on and finish my map, which ended up looking like this and went into his report on the observations:<\/p>\n

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I’ve used a hillshade <\/a>layer as a backdrop for two reasons – it’s a simpler layer that also allows our dear readers to get a sense of topography, and frankly, I didn’t have anything that worked better.<\/p>\n

Just to recap, the dragonfly data from Taveuni unfortunately ended up on opposite ends of the world.\u00a0 A Pacific centred projection helped matters by reuniting the torn asunder island.\u00a0 And it all ended well with a decent map for Milen’s report.<\/p>\n

(To be honest, I think I’d do a few things differently with the map if I had the chance, but there you go. IAh well, I must let it go, like a dainty dragonfly.)<\/em><\/p>\n

In another post we’ll go over how radar-derived elevation data from a space shuttle mission helped finish off this map.<\/p>\n

C<\/p>\n","protected":false},"excerpt":{"rendered":"

Spatial data of dragonfly observations from the Fijian island of Taveuni posed some interesting mapping problems .\u00a0 This post covers how we resolved them using a new projection. You may recall an earlier post where we talked about collecting dragonfly sightings across the Pacific.\u00a0 My colleague, Milen, is quite the dragonfly aficionado and has been […]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1947","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1947","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/comments?post=1947"}],"version-history":[{"count":1,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1947\/revisions"}],"predecessor-version":[{"id":4123,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1947\/revisions\/4123"}],"wp:attachment":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/media?parent=1947"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/categories?post=1947"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/tags?post=1947"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}