NYC Projected

NYC Projected

Much has been written about map projections and coordinate systems. I will not attempt to add to what has already been well covered by those with more technical expertise and writing skills than myself. If you are interested though the USGS is an excellent resource. You can start here.

I recently had a thoughtful email discussion with a colleague who questioned our (NYC DoITT) use of the New York State Plane coordinate system in this era of web mapping. This got my thinking. Did I really know why the choice was made? Do we just continue to use this coordinate system because we always have?

Unfortunately, none of the individuals that were involved in the first digital mapping effort to fly the City and capture planimetric data (more here) are around anymore. And there is no documentation on the reasoning that went into the decision. However it is fairly straightforward to work backwards to understand within reason why the coordinate system was chosen. The New York State Plane Long Island Zone (EPSG 2263) is a local coordinate system that provides a high degree of accuracy and balances size and shape well. I know with certainty that the flyover and basemap were developed in support of the NYC Department of Environmental Protection (DEP) sewer and water mapping initiatives. Taking a leap of faith one can presume that DEP required a degree of accuracy that was best supported by the linear accuracy of the New York State Plane coordinate system.

epsg-2263-labels

To maintain consistency, all subsequent flyovers and planimetric captures have used this same coordinate system. But the story does not end there. Updates have been made to the coordinate system over the intervening years. The High Accuracy Reference Network (HARN) introduced an upgrade to North American Datum of 1988 which was subsequently upgraded by Continuously Operating Reference Station CORS in 2011. These very slight adjustments lead us to a current EPSG 6539. We have yet to adopt this new coordinate reference system (CRS) due to the disruptive nature of the change for minimal difference, but may move to it in the future.

epsg-3857-label

State Plane is popular within state and local governments across the US but is that what the geo and open data communities are using? Most likely not. Google developed and popularized the ubiquitous web mapping coordinate system Web Mercator for its online maps. GPS coordinates and OpenStreetMap use WGS 84.

Thankfully transforming between coordinate systems is not as cumbersome and as problematic as it was in the past. Many systems can reproject  data on the fly. But is there a real difference between these popular coordinate systems used for NYC?

epsg-4326-labels

The three maps illustrate how the NYC landmass appears in the three coordinate systems discussed. State Plane and Web Mercator maintain the landmass shape well but the Web Mercator size is slightly distorted. This is most evident in Staten Island. The most extreme difference can be seen in WGS 84. The entire landmass seems stretched and flattened.

To answer my previous questions, I think the right choice was initially made and staying with the choice was an equally good decision. That said, we can support more than one coordinate system.

In the coming months, we plan on publishing our aerial imagery (digital orthophotography) as map tiles (WMTS) in both 2263 and 4326. So please check back. And feel free to leave us a comment with your thoughts on the topic.