Going on a fishing trip. Hope to give a report with pictures when I get back.

[bwelde]

The green lines are depth contours pulled from government data and overlaid on Google imagery. We go out and find this type of information for lakes across the country and then publish it online so you can see depth information on top of Google, Bing, USGS Topo maps, etc.

Are the green lines something your added to google earth? They look like tracks that were added using some of the software I've seen and used in the past.

I did the same thing with the Google Earth Map of Bluegrass Pit and Otter Pit back home where I live. It's neat to be able to add tracks from the Humminbird SI units.

I see my fishing spot where I know there are some huge crappie in the Early Part of April if the water's down. The big females staged in 12 ft deep water with timber standing in it and we caught them 8 ft deep.

[Moveon]

Hey bwelde:

I checked out your web site briefly and really liked the accuracy statement on your web site. You sum it up nicely. I wish that other map making companies would be so honest with their products.

I've been trying to get people that do these type of maps using GIS data and GPS/depth finder surveys to come clean about the accuracy of their maps. They give me some gibbersh about criss crossing the lake multiple times and try to tell me that this increases the accuracy of the GPS readings. I say that the maps positional accuracy is determined by the limit of accruacy of the gps readings. Your statement below talks about mathmatics and how that introduces errors too. Converting from one system to another system can throw mathmathical errors into the accruacy of the position like you say. And since the Earth is not perfectly ROUND the map datum has a lot to do with the accuracy of any GPS reading taken on this earth. GPS uses trigonometry to determine the angles and distances from the satellites to any position on the earth. but since the Earth is not round and trig used circles to determine the numbers the data will not be as accurate as it should be since the Earth is not a perfectly round circle. Like you said it gets pretty technical but the gist of the prolbem is not that hard to see and it must always be remembered that these digital maps are not perfect.




-------------------------------------------------------------------------------------------------------------------------------Positional AccuracyWhen you use the map you will find that some of the information does not always line up. The reason for this is how the information was created. There are many factors that go into creating map information, including funding. Generally speaking, the more funds an agency has at its disposal the more accurate development methods it can use.
For example, you may notice that some contours cross onto land. If the agency creating the contours performed on the water surveys the contours the positional accuracy would have been much better. However, that method is pretty expensive. It is much cheaper to draw the contours in a computer-based environment using other sources such as older hard copy maps. When this process is used it can introduce positional error to the data.
Other factors such as coordinate systems and data transformation play a role as well. We are not going to go into detail here as it is pretty boring and technical and we can’t imagine you really want to know it. But in brief, there are many mathematical computations to determine location on the globe. When different computations are applied to the data slight variations in position can occur.
Locations in the map are therefore generalized. The planning you do here will get you close to the area you need to fish. We cannot guarantee the absolute accuracy but we can bet you'll be within 10-15 feet. Generally speaking the imagery provides your most accurate base layer to mark positions with.

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The green lines are depth contours pulled from government data and overlaid on Google imagery. We go out and find this type of information for lakes across the country and then publish it online so you can see depth information on top of Google, Bing, USGS Topo maps, etc.

[bwelde]

Thanks for checking us out. Our philosophy is that something is better than nothing. Having 10 foot contours, even with some positional warts, and being able to view those depths on top of different types of imagery offers locational intelligence that you can't get from imagery alone or contours alone. If we find updated contour data that is more accurate (based on on the water surveys) then we replace what we had in the map previously with the better content so you automatically can use it. We do have actual on the water surveys in the map with 1 foot contours, although as you have mentioned even that data should be used with caution.

Our stance is that if people have a wide range of different types of information to choose from and they can combine that information in different ways, then they can make the choice about what is useful or not.

Hey bwelde:

I checked out your web site briefly and really liked the accuracy statement on your web site. You sum it up nicely. I wish that other map making companies would be so honest with their products.

I've been trying to get people that do these type of maps using GIS data and GPS/depth finder surveys to come clean about the accuracy of their maps. They give me some gibbersh about criss crossing the lake multiple times and try to tell me that this increases the accuracy of the GPS readings. I say that the maps positional accuracy is determined by the limit of accruacy of the gps readings. Your statement below talks about mathmatics and how that introduces errors too. Converting from one system to another system can throw mathmathical errors into the accruacy of the position like you say. And since the Earth is not perfectly ROUND the map datum has a lot to do with the accuracy of any GPS reading taken on this earth. GPS uses trigonometry to determine the angles and distances from the satellites to any position on the earth. but since the Earth is not round and trig used circles to determine the numbers the data will not be as accurate as it should be since the Earth is not a perfectly round circle. Like you said it gets pretty technical but the gist of the prolbem is not that hard to see and it must always be remembered that these digital maps are not perfect.




-------------------------------------------------------------------------------------------------------------------------------Positional AccuracyWhen you use the map you will find that some of the information does not always line up. The reason for this is how the information was created. There are many factors that go into creating map information, including funding. Generally speaking, the more funds an agency has at its disposal the more accurate development methods it can use.
For example, you may notice that some contours cross onto land. If the agency creating the contours performed on the water surveys the contours the positional accuracy would have been much better. However, that method is pretty expensive. It is much cheaper to draw the contours in a computer-based environment using other sources such as older hard copy maps. When this process is used it can introduce positional error to the data.
Other factors such as coordinate systems and data transformation play a role as well. We are not going to go into detail here as it is pretty boring and technical and we can’t imagine you really want to know it. But in brief, there are many mathematical computations to determine location on the globe. When different computations are applied to the data slight variations in position can occur.
Locations in the map are therefore generalized. The planning you do here will get you close to the area you need to fish. We cannot guarantee the absolute accuracy but we can bet you'll be within 10-15 feet. Generally speaking the imagery provides your most accurate base layer to mark positions with.

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