EP60搭配服用地圖：弓頭鯨遷徙路線與阿拉斯加捕鯨聚落

原本只是抱著好奇心想去看看Nalukataq而飛到Utqiagvik (Barrow)，沒想到卻不小心入了個大坑。

在事前做功課時，就聽說每個聚落可以捕鯨的時間不太一樣，有的地方只有春天能捕鯨、有的地方捕鯨季是秋天，還有一些地方春天秋天都能捕鯨，但究竟為什麼會是如此呢？究竟是純粹因為各地生活習慣不同，還是有其他原因？

一直到離開Utqiagvik的那天，這個問題仍未獲得解答，於是我回來之後繼續尋找答案；以前求學階段非常不喜歡讀生物的我，還為此去鑽研了一下弓頭鯨的習性與遷徙行為。最後在看到遷徙路線圖的那刻，我整個恍然大悟——

原來，弓頭鯨的在春天和秋天的遷徙路線是不一樣的！大體上來說，位於西側的聚落只有春天會有弓頭鯨經過，因此捕鯨季為春季，東側的聚落則是在秋季；至於夾在中間的Utqiagvik，地理位置得天獨厚，因此春秋兩季都可以捕鯨！

這張圖上分享的資訊，不只解答了我自己的問題，也再次讓我感受到人與環境互動的迷人之處——環境當然不會100%決定人的行為，但環境條件的限制（無論是地形、氣候還是生物習性）仍然形塑了許多地方的文化傳統；認識這些傳統，不只讓我們更加欽佩老祖宗的生存智慧，似乎也對自然界的奧妙更加崇敬！

資料來源為Alaska Eskimo Whaling Commission
底圖為Esri World Oceans
投影法為North Pole LAEA Alaska (EPSG:3572)

【EP60 北極圈裡頂天立地的他們，稱呼自己為「真正的人」——弓頭鯨、極地民族Iñupiat與美國最北邊的小鎮】

🔈 收聽節目：
Spotify Apple Google SoundOn Firstory Baabao KKBox
各大播放平台搜尋「旅行熱炒店」或至官網

#podcast #travel #播客 #音頻 #自助旅行 #中文podcast #台灣podcast #阿拉斯加 #美國 #北極 #極圈 #永晝 #愛斯基摩 #因努伊特 #原住民 #捕鯨 #弓頭鯨

What Are Ground Control Points (GCPs) and How Do I Use Them?
A Guide to Using Ground Control Points with Drone Mapping Software
If you work with drone mapping software, you’ve no doubt heard talk of ground control points (GCPs). Used often in the surveying industry, as well as in virtual design and construction, GCPs greatly increase the global accuracy of drone maps. Although they are not necessary in every situation, GCPs are a vital tool for precision mapping. But what exactly are ground control points? And how do you go about using them correctly?

To help crack the code on ground control points, we’ve put together this short guide to using GCPs with drone mapping software.

What Are Ground Control Points?
So what exactly are ground control points? Ground control points are large marked targets on the ground, spaced strategically throughout your area of interest. If you use ground control points with your aerial map, you first need to determine the RTK GPS coordinates at the center of each. (We’ll explain how to do this a little later.) The ground control points and their coordinates are then used to help drone mapping software accurately position your map in relation to the real world around it.

It might be helpful to think of your GCPs as a series of thumbtacks placed on your drone map. Because the drone mapping software knows the exact location of each of these “thumbtacks”, it can reference their locations when it matches up all of the other points on the map.
When and Why Are GCPs Important?
When used correctly, ground control points greatly improve the global accuracy of your drone map. That is to say, they help ensure that the latitude and longitude of any point on your map corresponds accurately with actual GPS coordinates. This is important in situations where precision mapping and true global accuracy are needed. As we mentioned above, surveying companies generally use GCPs, because a high level of global accuracy is important in most of the work that they do. Virtual design and construction is another sector that often requires this level of precision drone mapping.

Landpoint, a surveying company based in Louisiana, uses ground control points when creating drone maps used for topographical surveying. Using GCPs on an 85-acre map, their team conducted an accurate aerial survey, saving over 80 man hours compared to traditional land survey methods.

Each drone mapping project is unique, and not all projects require a high level of global accuracy. Because of this, it is important to assess each project individually before you decide to take the extra step of using GCPs. But generally speaking, projects like geo-referenced overlays, design documents and land title surveys benefit from the use of ground control points. In an upcoming post, we’ll take a deeper look at which types of projects are best suited for using GCPs.

How to Construct a Ground Control Point
There is no one right way to make a ground control point. One important thing to remember is that the GCP must be easily visible in your aerial imagery. This is achieved by using high-contrast colors and by making sure the ground control point is large enough to be seen from your particular flight altitude. We generally recommend flying at 300 feet with a frontlap and sidelap of 70/75 when using ground control points. Keep in mind that this may change dependent upon the area you are mapping. Learn more by reading our recent post about mapping accuracy and reviewing our GCP support documents.

A number of companies do sell pre-made, portable ground control points. However, many drone users simply fashion their own.
This well constructed GCP was spray painted onto the concrete using a stencil. Notice that the marker is large enough to be visible from far away. A center mark helps eliminate any confusion as to where the center point is located.

If you’re unable to mark GCPs with paint, there are a variety of low-cost ways to make markers with items available from any local hardware store. The weather-resistant rubber and vinyl markers seen above end up costing about $5 each and are very durable.

Measuring the Location of Your GCPs
As we mentioned above, it is important to measure the GPS coordinates at the center of each ground control point. To do this, you need either a Real Time Kinematic (RTK) or Post Processing Kinematic (PPK) GPS receiver. Trimble and Leica products are commonly used for high-accuracy GPS measurements. New, lower-cost alternatives have recently come onto the market as well. Hiring a surveyor to measure the location of your ground control points is also an option.

Do not use a phone or tablet to measure the location of your ground control points. The accuracy of these devices is very similar to that of a drone’s onboard GPS system and will not deliver precise results. Instead, use one of the previous methods listed above, such as an RTK or PPK GPS receiver.

Best Practices for Using Ground Control Points with Drone Mapping Software
Of course, if you use ground control points with drone mapping software, it is important to use them correctly. Follow these best practices to help ensure your GCPs serve their intended purpose and improve the accuracy of your map.

Use a minimum of 4 large GCPs: DroneDeploy requires a minimum of 4 ground control points. Each should measure at least four feet. No more than 10 are usually needed for larger maps.

Evenly distribute your GCPs on the ground: For most maps of moderate size, we recommend 5 GCPs, one located near each corner and one located in the center, as pictured above. Also, make sure GCPs are spaced far enough apart, to avoid confusion. As a general rule, if you can see more than one GCP in an image, they are too close together.

Create a buffer zone around your map’s perimeter: We recommend a buffer zone between the edges of your map and any ground control points. This ensures there is enough image coverage to carry out reprocessing. The size of your buffer zone should be somewhere between 50–100 feet, depending on the overlap of your flight. A higher overlap produces more images and generally requires less buffer zone.

Be aware of elevation changes: If the area being mapped has noticeable elevation changes like hills, mines and valleys, make sure to place at least one ground control point on each of the different major elevations.

Make sure your GCPs are unobstructed: Visual obstructions like overhangs, snow, shade or glare make ground control points difficult to identify on your drone map.

Know Your EPSG code: Before processing your map in DroneDeploy, you must enter the EPSG code that relates to your GPS measurements. Choose your EPSG code by modifying the settings of your GPS measurement device. In most cases, we recommend using WGS84 (EPSG: 4326).