Digitizing LWC Flashcards
Why digitize? Why care?
1) If you don’t have or cannot obtain the digital data you need, then you must create them in the field or from other source materials – unlike the academic situation of this class, most projects cannot rely on the ability to download suitable data.
2) Most geographic variables change over time, some quite slowly, some quite rapidly.
3) Data often need continual updating to remain useful. Even if the original data are available, the updates may not come along as often as you need.
4) Many common existing sources are reference maps and images
5) Maps have been around a long time and widely adopted. Images are also well understood geometrically so they can be converted to constant scales like maps (more soon).
6) Their strengths and weaknesses have long been understood:
- Generalization, as mentioned in previous lectures – a lot is left out.
- Distortion with age or error.
- Changing standards for classification and depiction
What scale are topographic maps of the land in USGS? What are they good for? What are they not useful for?
1:24,000, 1:62,500, 1:100,000, or 1:250,000 scale.
Excellent for moderate to small scale data.
But not useful at scales larger than 1:24,000
for example, urban development.
Also, the data on these maps may be ancient for project needs (40-50 years old in smaller scales)
again, urban development as an example.
What are aeronautical charts?
Topographic maps of the skies and protruding land surfaces of the US (NOAA) – very up to date (57 day cycle)
What are nautical charts?
Topographic maps of the coast, harbors and waterways of the US (NOAA) - also very up to date.
What are the scale issues with the topographic maps?
We have a lot of extremely useful common national databases in the US, but we still may not have what you need for a local project in which you need larger scale, or more detailed, or more recent data. A good example of a useful layer for a GIS in local use is the cadaster – parcel tax map.
What is digitizing?
Converting to digital. Digitizing converts an illustration to a digital form in two or three dimensions, removing annotation and other stray marks, and keeping only the boundaries (vector). We can take two approaches to getting digital spatial data, digitizing or scanning.
Slide 12 for illustration.
How do you georeference?
First, Georeference a map, scanned image of a map, or image to the desired coordinate system.
Find points that you know the location for in the desired coordinate system
Locate them in the local digitizer tablet or screen pixel system
Compute an equation that converts one to the other (affine transformation)
What is an example of georeferencing with a grid?
Slide 14.
How do you georeference without a grid present?
Process is the same, but the difficulty is that there is no grid marking on the image usually to help you. Look for points you could find either on a map of the same area, or on the ground in a field visit to the site with GPS
What is the objective of the georeferencing table?
Objective is to find a set of transformations: rotation, scaling, and translation that make the values in the map columns equal the values in the UTM columns of the table
Slide 16 for image.
For georeferencing entry you have to choose between ______ or ______.
Vector or raster.
What is vector for georeferencing entry?
Digitizing is a vector approach to map capture that involves a tablet or is done on-screen
For vector data entry what does line sampling during digitization involve?
Decisions. Lines contain an infinite number of points as we said earlier, the vector model samples from those points. Your digital line is a sample of the original based on a few selected (X,Y) locations (points).
But, how many points?
-Scale
-Purpose
Slide 18 for illustration
If all points are accurate, then the more points the longer the line gets, but it never reaches the actual length unless it is a ________.
Straight line
All lines are shorter in their digital representations than they are in reality. Lines that are \_\_\_\_\_\_ are more affected by this issue A)Straight B)Somewhat irregular C)More irregular D)Extremely irregular
Extremely irregular
