We want to map a parcel of land forest on then mountain, which differences in elevation among land parcel . What method to measure and to map/delineate the parcel ?
If you have Ipad or other perhaps, you can download the Avenza program app. and if you have a georeferenced map of the area or have a friend in GIS download a georeferenced map in ARCMAP, and there are a few more steps, you can go to the field with the ipad locator turned on, upload the georeferenced map into Avenze and you can see where you are walking hopefully assuming there is some satelite coverage to locate your ipad, and you can walk around the parcel, locate points on the map. The uploaded map from ArcMap may be a topographic map, aerial photo, or whatever combination of layers you want. You can attach and locate pictures if you want of your trip of various sites, big trees, boundary markers, streams, etc. There are a few papers on the internet I think too on using this for fire or timber proposals. I have used it a few times, and definately great to locate yourself and can even be used for road surveys, locating stream crossings, erosion problems, big trees, ponds, wetlands, rare plants, old mining sites, or other interests.
This is an example of a fire application of this, I have not read through it and there are more on the internet of how this has been applied. The app is free and I assume it would probably work everywhere that the Ipad locator would also work.
http://www.pobonline.com/articles/97911-us-forest-service-launches-avenza-pdf-maps-app-for-firefighting
It depends on what accuracy you are looking at. If you are good with an horizontal accuracy of about 3.5 meter your are good to go with the described method above. There are however two limitations to it:
1. Vertical accuracy is much less then the horizontal accuracy. So if you need proper elevation readings, a "normal" GPS device is very inaccurate.
2. You mentioned two parameters mountains and forest - both will lower the accuracy significant due to covered horizon and reflection of the GPS signal.
There are some technical solutions to increase the accuracy of GPS through post processing of your data and / or real time correction.
Hi Vo Quang,
1) Acccording to me you first have to know the resolution of your original image. 2) Knowing that, you have to look for a DTM with the same resolution (or close to that of the image).
3) Next step is to collocate the DTM with the image.
4) Once done, you can determine elevation and distances projected on the horizontal plane parallel with the ASL plane, or along the height gradient plane (slope plane).
5) To delineate the parcel an calculate its surface area, you can do exactly the same. Project the surface on the plane parallel with the ASL plane or along the height gradient plane (slope plane), which will be different from the ASL plane according to the slope angle (determined with the DTM).
I figger this answers your questions? Its just a bit of GIS theory.
Cheers,
Frank
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