# Why can't I exactly match the same points on different VFR sectional charts?

I am taking a trip from somewhere on the Phoenix sectional up to the grand canyon. I wanted to just splice the two sectionals together on my computer and then print them out.

I picked a coordinate to align (as you can see in the first image), and with that coordinate aligned, almost nothing else on the map matched up.

Why aren't the sectional maps standardized to people can do this more easily? I would have loved to make a "custom" map out of the Phoenix sectional, Vegas sectional, and Grand Canyon map, but it's virtually impossible without modifying them further, which I don't want to do in case of messing up the scale and/or orientation.

• Sounds like a map projection issue: what you're trying to do would be fine if we lived on a flat earth but it doesn't work on a sphere. Apr 6, 2015 at 22:04
• But the maps take into account the curvature of the Earth, right? Apr 6, 2015 at 22:05
• @SpongeBob that only goes so far Apr 6, 2015 at 22:06
• So how does skyvector.com make everything match up? (Which I now realize isn't perfect, but it's pretty close) Apr 6, 2015 at 22:09
• Presumably it stores the map in spherical coordinates, and projects to rectangular on an as-needed basis.
– Mark
Apr 6, 2015 at 23:12

This is by no means specific to aviation charts. Most likely it's simply general property of local maps.

Maps are flat, but earth surface is (approximately) spherical. So to represent ground features on the flat map a projection is needed and this projection can't preserve all relations—angles, distances and areas—precisely. So there are many different projections that distort different properties.

Small area maps are usually drawn in azimuthal projection with the central point in the centre (or significant point near the centre) of the mapped area. While such projection does not preserve exactly any properties out of the centre, the distortion grows with distance from the centre, so for small areas it's appropriate.

This however means, that each section uses different projection and that near the edges the maps will be distorted each in a slightly different way. That's why the maps can't be easily laid out to form a bigger map. You would have to lay them out on a sphere.

You could try using some software for stitching panoramatic photos (here is one such you can use for free). That should be able to apply suitable transformation to make the maps match. At the cost of increasing the distortion.

• According to Wikipedia - "The US systems of VFR (visual flight rules) sectional charts and terminal area charts are drafted on the [Lambert conformal conic projection] with standard parallels at 33°N and 45°N." - so I'm still surprised adjacent maps, at the same scale, can't be joined. Apr 7, 2015 at 13:22
• Hm, actually looking at the screenshot it really seems more like imprecise drawing then projection difference, because some lines are offset to one side and other to the other. And projection difference would have them skewed the same way with the difference increasing from the reference point. Apr 7, 2015 at 14:33

The FAA's graphical index of sectional charts shows all the charts for the coterminous 48 states laid out on a single map projection, but the areas covered by the charts do not all align with the rectangular boundaries of that projection.

If you were to obtain copies of all the charts and attempt to lay them out according to that projection, you would have to rotate the western charts clockwise and the eastern charts counterclockwise. Any two contiguous charts might or might not be rotated at the same angle; charts directly north or south of each other (several pairs along the northern edge of the 48 states) might be aligned, but other pairs of charts that have an east-west offset (most of the other adjacent pairs) are likely to be rotated at a different angle.

So even if the charts are truly drawn to the same projection and are perfectly executed, you will not be able to align them perfectly just by moving them up, down, left, or right. You will need some rotation as well.

There are a few reasons that may cause this. First off I assume you are getting your images from here in which case if you look at the 2 PDF downloads in question the maps were issued on different dates. Although its unlikely it is possible that things were moved on the map between these 2 issue dates.

I cant tell where you are trying to join the maps but if you are trying to join them mid map (not edge to edge) you may have issues even though they should line up. This is a decent write up of the different kinds of map projections and how distortion comes into play when projecting a spherical surface on a flat piece of paper. This distortion could cause some of the issues. I would also thing some error can end up in maps when they are printed. If you use apps like foreflight or the like you will see that the points which are often GPS based dont always line up perfectly with the map.

Lastly, you should NOT make your own maps to be used for actual navigation. There are many issues this could create that I will not go into as it does not have to do with the answer to the question.

WAC and sectional charts use Lambert conformal conic projections. the scale error varies with lattitude to preseve great circle as straight lines. Cones have flat edges sphere's do not. https://en.wikipedia.org/wiki/Lambert_conformal_conic_projection