TransDEM Forum

Hello all!

I am currently working on a NYCTA layout and am collecting the georef map images from the Map Tile Servers. I am using the Open Street Map, and the zoom level is about 16-17.

Prior to updating the newest release of TransDEM, my map images were flawless and seamless. Now, whenever I go to add a new image, the maps come together choppy--you can see a thin white outline where a new map as been added. It looks like a kindergartner's bad copy and paste job. Here are two examples. One is a little zoomed out to show multiple choppy parts, and the second photo is a close up of an area with the choppiness still visible.





Can anyone shed light on this new, (and mildly irritating) issue?

Thanks!

-A

Well, nothing has changed here really and I don't thing there's anything to worry about. I'll try to explain what happens.

Each clipping acquired from a map tile server is a bitmap filled with a number of 256 x 256 pixel images. The images represent a map or orthophoto in standard Mercator projection (i.e. with the main rotation axis through the poles) and projected from a sphere. TransDEM uses a different projection, internally and when exporting to Trainz. This is UTM, the Universal Transverse Mercator, with the axis through the equator and the zonal system, projected from an ellipsoid.

Each clipping acquired must be converted standard Mercator to UTM. This is a non-linear transformation and will introduce error, as it applied to pixels of an raster image. The error will normally +/-1 pixel, a bit more at times, depending on where the rounding takes place.

As all clippings are converted individually (and remain individual images after conversion) they will join to their neighbours with a marginal but noticeable seam.

When viewed in TransDEM, this seam may become visible at certain zoom levels and invisible at others. This has to do with the rounding to whole pixels again. The TransDEM renderer cannot display fractions of a pixel.

Thanks for the information!

-A