How to define the number of meters per pixel

[MeganDecker]

I am creating a map and I need to tell ArcGIS that each pixel is 135 meters across so that I can take some measurements. To be more specific, I have an image taken by a spacecraft of another planet's terrain. I basically need to tell ArcGIS what the resolution of the image is. How do I do this? Thanks in advance!

[RobertBorchert]

Which planet?

There are a  number of projections out there for different planets.

then you can just project the image and use the regular measurement tool

I am creating a map and I need to tell ArcGIS that each pixel is 135 meters across so that I can take some measurements. To be more specific, I have an image taken by a spacecraft of another planet's terrain. I basically need to tell ArcGIS what the resolution of the image is. How do I do this? Thanks in advance!

[MeganDecker]

I am mapping a volcano on Io and I have tried using the correct projection. I am going to measure the shadow to see how tall the scarp is around the volcano. I know that it is ball park range of 900 meters but when I tried to measure while the image was under the Io projection I am getting measurements that are way too small. Like 14 meters or so. Am I missing a step or doing something wrong?

[MeganDecker]

Okay so I measured the image with the measure tool and it said that it was 735 by 680 meters and the image is 735 by 680 PIXELS when I look at the image properties and I double checked with NASA's JPL website. So I think that Arc is already measuring in pixels, it's just saying that it's in meters. So now I should just have to measure the shadow which will give me the number of pixels and then subsequently multiply that number by 135 meters/pixel to get the height of the scarp. I think I've got it figured out. Thanks for your help. (P.S. let me know if I am way off!)

[RobertBorchert]

When you click the measurement tool on the right is a drop down selection

Planar, Loxodrome, geodesic and great eliptic.

By default it should be set to Planar.  Try the other 3 out.

Okay so I measured the image with the measure tool and it said that it was 735 by 680 meters and the image is 735 by 680 PIXELS when I look at the image properties and I double checked with NASA's JPL website. So I think that Arc is already measuring in pixels, it's just saying that it's in meters. So now I should just have to measure the shadow which will give me the number of pixels and then subsequently multiply that number by 135 meters/pixel to get the height of the scarp. I think I've got it figured out. Thanks for your help. (P.S. let me know if I am way off!)

[MeganDecker]

Thanks! I'll give that a try.