8 replies to this topic

[Carri Marschner]

Greetings,

I am mapping research data on ~5-hectare plots on steep terrain. The lead scientist has pointed out that our lovely GPS data does a great job of putting our data on a map, but what he needs to know/see is the distance over the ground between our data points, the way a mouse or other ground animal would see the world.

I'm not sure how to approach this, as I have worked entirely in normal, bird's-eye mapping. I don't think GIS can unhook from real geographic locations and warp the data so that ground distances are shown. Is this correct?

My current idea is to use 3D Analyst to generate over-terrain distances between points using the best DEM-generated elevation data I can find, export that into... Excel? and do a basic grid of the research elements. I realize this is cartographically neanderthal, but I'm an extreme newbie. Does this make sense? Is there a better way to do this?

The tools we have in hand are GIS ArcInfo 9.3 and any free or very low-cost software that may help. Any advice or input would be gratefully accepted.

Carri Marschner
CIES, Millbrook, NY

-draft of plot elements attached

Attached Files

•  Teas_GPS_data_May_2010.jpg   712.2K   73 downloads

[David Medeiros]

I'm not sure of the exact work flow but if you have an elevation layer associated with your project you should be able to get both birds eye and real distances from point A to point B in Arc.

Edited to add: As I read your post again it seems your also concerned with visualizing the change in distance not just gathering the distances themselves. Draping the points over an elevation surface wont change how they appear when viewed from directly above. You could alter each points location by some factor that reflects its distance from its neighbors so that true distance along the ground turns into visable distance varations when viewed from above, but then your points will no longer match the air photo image or background elevation layer.

Can you simply gather the real ground distance and add it as a annotation layer to be labeled in the final layout?

[dsl]

If you are looking to visualize how the points and lines sit on the terrain. You can "drape" these in KML/Google Earth. Here is an example of a tessalated line in Google Earth. You need the google earth plugin to view it. The terrain data supplied isn't the best, and probably not at the level of detail a mouse would experience.

If you have 3d Analyst, then there is an equivalent using ArcScene. Just open the DEM and the points/lines. Set the DEM properties so that it uses itself for the elevation, and the points/polyline properties use the DEM for their elevation. Or if your GPS captured the elevation data, you can use that instead. I can't remember the exact terms ArcScene uses.

Cheers,
David

[Nick H]

The lead scientist has pointed out that our lovely GPS data does a great job of putting our data on a map, but what he needs to know/see is the distance over the ground between our data points, the way a mouse or other ground animal would see the world.

According to Peter Guth (the mastermind behind the MICRODEM project) the differences are not as great as might be expected. In the MICRODEM help file he says:

"Many people feel that the map distance "as the crow flies" will significantly underestimate true distance. This will generally not be true; I once did a numerical analysis of the feature at West Point, NY (fairly mountainous for the East coast), going both parallel and perpendicular to the grain of topography, and the map distance varied from the actual slope distance by only a few percent. Unfortunately, I do not have a copy of that analysis. Your actual walking distance may be significantly greater because you don't traverse a straight line, but tend to follow a switchback trail, but that is another matter".

"The following chart emphasizes the point. The distance or area will be increased by the secant of the angle of slope, and until you get to very large slopes, the secant is essentially 1. Even in steep areas (and 45° is about as steep as you get in nature on what looks like a vertical cliff), only a small percentage of the area is that steep". [Chart not reproduced here]

Attached, an illustration showing a track drawn by hand in MICRODEM on a rendered DEM for somewhere in Wales.

Regards, N.

Attached Files

•  track.jpg   157.54K   74 downloads

[BioGeoMan]

I would have to agree with Nick. You are essentially covering hundreds of meters and the differences between ground vs. air in straight lines is minimal at this scale. What should be more interesting is the path that the ground critters take as I am sure they do not travel in straight lines. But if you are assuming straight line travel, then slope on this small of scale should not affect distance in a significant way.

[Carri Marschner]

Thank you to all of you for your thoughtful responses. The line draped over the Grand Canyon was particularly cool. :)

I appreciate that the difference between topographic and direct distance are minimal in most cases. Unless I'm understanding the math wrong here, in this particular case we are looking at distances between data points of 15m, some of which sit on slopes that have 10m elevation gain, which causes our 15m distance to look like (and measure in 2d like) 11m. (a^2 +b^2=c^2 with a 10m elev change over 15m = 11.1m horizontal distance?) This throws our grids out of square and skews our data results on these steep slopes.

Having looked further into 3d Analyst following remarks made here, I think draping our interpolated data over a 1m DEM will give me the basic information I need, provided I can get 3D distance measurements out of 3D analyst in ArcInfo. Regarding visualizing the data in 'flat': I will suggest to the powers that be that David Medeiros's suggestion of annotation is the best solution.

Thank you for your help; if you have additional suggestions I would welcome them.

Carri Marschner

Attached Files

•  Demo_of_plot_slope_spot_13.jpg   66.76K   29 downloads

[SaultDon]

Thank you for your help; if you have additional suggestions I would welcome them.

Seeing as you have access to an ArcINFO license and the 3D Analyst Extension.

Go ahead and use the Surface Length (for arc version 9.3.1 and earlier). For v10, that tool is now deprecated and replaced with the Add Surface Information tool.

[Kathi]

I appreciate that the difference between topographic and direct distance are minimal in most cases. Unless I'm understanding the math wrong here, in this particular case we are looking at distances between data points of 15m, some of which sit on slopes that have 10m elevation gain, which causes our 15m distance to look like (and measure in 2d like) 11m. (a^2 +b^2=c^2 with a 10m elev change over 15m = 11.1m horizontal distance?) This throws our grids out of square and skews our data results on these steep slopes.

Do I understand correctly that you have two points which on a 2-D-map are 15 m apart, and one of them is 10 m higher than the other? Then you have a slope of roughly 42 degrees, and the true distance between your points in 3-D is 18 m (square root of (15^2 + 10^2)).

I agree that in moderate terrain conditions the effect of elevation on true distance is minimal. But 45° is steep but still nothing near vertical. Especially in a mountainous area you could easily have slopes that are quite a bit steeper (when you look at extents of some hundreds of metres). If your slope is 45°, your true distance increases by a factor of 1.41 (square root of 2), which is not all that insignificant, I think.

[ravells]

Could you get it without having to use maths?

Take a screenshot of cross-sectional view of the path you want to follow and use illustrator to auto-trace the image.

Add a new node to the start point and the end point of the distance you want to measure and break the path at those nodes.

There is a plug-in called 'snap measure' for illustrator which will measure the distance of the path. You'll probably need to to have a scale bar with your exported cross section so you know what to multiply the path length by to get the right value.

Another way might be to stroke the path with a dash pattern (with each dash and gap of a known length) and then count the dashes and gaps?

Oh, just noticed that you posted your question in May this year so you've probably long moved onto other things!