Hello,

This is a follow up from an old thread.

http://69.17.46.171/forum/t37681.2

I have a map of a site with MS VE imagery linked. I also have parcel and other site data (buildings, infrastructure, etc.). I have separate maps created with layouts for each. One for the aerial imagery and site data and the other for just the site data.

The aerial imagery has a Mercator (meter) projection. I didn't change or alter the projection and made maps based on this project. I made maps to fit a specific scale (1"=50'. I made a view with 1:600 and used that in the layout).

The site data map has a NAD 83 (foot) projection. This data was imported with the projection and hence was retained throughout the map making process. I used the same process as described above to make the layouts.

I discovered that there are differences in both maps. When scaled the same, the aerial map is stretched. I have a building that measures 500' in the site data map (which is correct) and that same building is stretched to 600'. I tried re-assigning the project of the aerial imagery, but then it moved way out in space.

Patrick

Patrick,

Are you measuring points based on the imagery? Remember both VE & GE are poorly geo-referenced. They are basically accurate to USGS quads if not worse. A lot of the data I've used in the California was in the Albers projection...could that be the case for your data?

Also, have you set your units properly? I know what is displayed sometimes can be different than expected...haven't messed with it lately so i'll defer to more experienced users on those issues.

Good Luck

Hey Patrick,

Could your distance measurement problem be related to the issue brought up here?

Chat with Manifold users on IRC Server: irc.freenode.net, Channel: #planetmanifold, IRC FAQ

Yes it related. Measuring within the context of the map and using CRTL helps. But, taken one step further and creating the layouts (paper maps), the scaling in the layouts do not match. The scale bars match, but the same data (ie building) does not. Is there another magic switch I'm missing?

I'm using English units set in Tools>Options. The NAD 83 map is set in feet. However, the VE map is set in meters. I don't know if this has anything to do with it. I do understand that aerial imagery (VE or GE) are poorly geo-referenced.

Hey folks,

I wanted to update you on how I was able fix or at least mitigate the issue. I cheated.

I went into the map of the aerial image and started with the saved view for what I thought was my preferred scale. I then started to tweak the scale ratio in the zoom box on the tool bar with manual inputs. Then I saved the view and loaded it on the layout. I knew what my reference dimensions were in the other layout that scaled correctly. I just measured (using the tracker) my known lengths. It was a trial and error process to get it within some acceptable difference. The final outcome was acceptable.

I know. That's probably not the best use of the software, but given the time constraints that was the best solution.

If anyone knows another work around, I'm all ears.

Patrick

You never get a "real scale"-map in mercator. The problem is the mercator projection is a projection for the whole world and so you get a distortion.

In Mercator-projection you always get a distortion, the only way is unlink, download and change the projection.

Look the example in attachement....

Have you looked here?:

http://en.wikipedia.org/wiki/Mercator_projection

Google Maps currently uses a Mercator projection for its map images. Despite its relative scale distortions, the Mercator is well-suited as an interactive world map that can be panned and zoomed seamlessly to local maps^{[citation needed]}. (Google Satellite Maps, on the other hand, used a plate carrée projection until 2005-07-22.)

The Google Maps maximum latitude φ occurs at ±85.05113 degrees when the Mercator y value = π. Or more precisely:

Attachments:
distortion VE.pdf

Adding a Postscript:

http://cfis.savagexi.com/2006/05/03/google-maps-deconstructed

http://www.sharpgis.net/category/Virtual-Earth.aspx

(at the end of the page...)