Digital Map Limitations

[mhabiger]

Lots of folks run a digital mapping app these days. One thing the mapping companies (including mine) haven't been great at is discussing the limitations of the technology. So I put together a video discussing 3 of the main limitations of mapping technology when it comes to knowing where you are in relationship to property boundaries. With the corner crossing ruling and what feels like a growing predilection of folks to hunt boundaries, I figured this might help folks think more critically about how they use the apps and how they use the information on the ground.

For those who don't want to watch the video, here are the 3 limitations:

1. Horizontal Error in Imagery - the imagery (or topo maps for that matter) have error. When a plane or satellite (depending on the source) captures the imagery and then that is projected to the earth's surface, there can be several meters of error in placing that imagery. It all depends on the number of ground control points and resolution of imagery captured. In general, rural areas won't be as accurate as urban areas because a lot of providers capture higher resolution imagery in urban areas.
2. Error in Property Boundaries - every digital mapping provider sources property boundaries from a 3rd party who aggregates them from county and local governments. The accuracy of the boundaries varies from county to county. In general these cadastral databases are not produced through boots on the ground surveys of every parcel. So the accuracy varies by the method used to create it, number of control points used and if any survey data is incorporated to the files overtime.
3. Device GPS Measurement Error - this is one most folks probably are aware of. Your smartphone gps is going to have varying accuracy depending on number of satellites and obstruction of field of view (i.e. are you in a steep canyon, heavy canopy cover, etc...). In general you can assume around 5 meter accuracy on smartphone gps.

Personally, when I'm in the field near boundaries I like to stay about 10 yards away if there are no visual markers of a potential surveyed boundary.

 

[TheYukiYama]

My family has land and we have people hunt right up along a fence even though when we had it surveyed the fence was technically about 10 yards onto our property. I couldn't ever imagine trying to give a guy a hard time for that. The county plat map with satellite shows the boundary a bit in between the surveyed corner and the fence. Again, I won't ever be giving anyone any grief.

I tend to take the same liberties with others property. I view it as if they are that concerned with the exact line, then they should have it surveyed and clearly marked. I understand this isn't always the legal understanding depending on the state, but that's what I believe in and will continue to do. (Most states I spend time in do require posting or notification to enforce trespassing). If it ever becomes an issue, I'll deal with the legal ramifications at that time. c'est la vie

 

[Chris in TN]

I worked in the GIS field for almost 20 years and still work in a field that puts me using GIS data multiple times per day and I was absolutely amazed at how people took to the various public-facing GIS data products and how they always assume the data is both positionally and legally perfect.

Always remember - all models are incorrect. some are useful.

 

[Still Hunter]

A ranch in Colorado bordering the National Forest where I often hunt recently put up a new fence. As a bowhunter I try always to stay 100 -150 yards off any property lines. However, they cleared all across that mountain and up those side hills for that new fence. So now I have gone right up that fence line several times now as it saves a lot of time. A bit brutal uphill but sure saves some time. I have no intention to ever shoot over that fence but I sure like that new wide path! One I get up the elevation I want I will break off and head on where I want to go. I have many corner waypoints marked on my OnX to make sure I do not get close to property lines. I have also learned that "All Trails" does have at least one trail marked a little better than OnX. I was worn out one night ( about 11:00 pm) trying to get back on a main trail after I had been pathfinding up several miles up a mountain. OnX was about 50 yards off the trail. Heavy thick timber. I zigged and zagged and was just getting really frustrated. was close to just sleeping right where I was and making camp for the night. I knew my hunting partner would already be worried and I was on the wrong side of the mountain, no where near any cell service and 3 1/2 miles from main camp. The trail actually made a big jog in that location and I was getting pretty bewildered. As a last effort I pulled up "All Trails" and it showed a big jog in the trail right where I was at, that was not on the OnX map. It Indicated the trail further to the Northwest so I went for it and sure enough I found that trail. 50 yards made a huge difference in that thick stuff. That jog was the only part of that trail that OnX did not have correct. "Pobody is Nerfect !" and neither is all this new technology. But is still nice to have it available for us.

 

[Coveyleader]

Number 1 these days is grossly incorrect depending on who captured the imagery, and how. Today’s imagery can be down to the CM level. As in less the 2-3 inches depending on the work, but I agree most sourced imagery along with phone GPS (not using a correction source), 5 meters is pretty safe “for the most part”

 

[mhabiger]

Number 1 these days is grossly incorrect depending on who captured the imagery, and how. Today’s imagery can be down to the CM level. As in less the 2-3 inches depending on the work, but I agree most sourced imagery along with phone GPS (not using a correction source), 5 meters is pretty safe “for the most part”

You are confusing image resolution with horizontal accuracy. The resolution of a pixel might be 15cm x 15cm but the coordinates of that pixel on your map to the actual location on the surface of the earth are typically off by several meters.

 

[Coveyleader]

You are confusing image resolution with horizontal accuracy. The resolution of a pixel might be 15cm x 15cm but the coordinates of that pixel on your map to the actual location on the surface of the earth are typically off by several meters.

No, I’m referring to on the ground accuracy. I deal with it daily.

 

[mhabiger]

No, I’m referring to on the ground accuracy. I deal with it daily.

Then you are using different providers than any of the consumer hunting apps. We all use google, mapbox or esri. They purchase imagery from planet labs, maxar and vexcel. I've talked to all of them and not one of them claims to have greater than several meter accuracy.

 

[Chris in TN]

The resolution of a pixel might be 15cm x 15cm but the coordinates of that pixel on your map to the actual location on the surface of the earth are typically off by several meters.

Start with the error inherent in most digital elevation models then add the error in tying aerial imagery to ground control points - or stretching all of that over a DEM - and next thing you know you're looking at a mountain wondering why that alpine lake is slanted downhill.